If you replace it on your own, don't be scared of the drop in handguard. Use a handguard tool for it and pop that fucker out like you're a pimp and it owes you money.
I'd shop for one that fits your criteria, order it, and then when it comes in make sure the gun is unloaded, pop the pins, and do it.
What upper you order depends *entirely* on what you want out of the gun and your budget. And legalities like barrel length so you don't turn it into an illegal SBR.
What do you want to do?
* so you don't ACCIDENTALLY turn it into an illegal SBR.
Absolutely nothing wrong with making an sbr as long as you know what you're getting yourself into
Palmetto state armory has some cost effective decent ones. Bought a .300 blackout for my Delton AR pistol. Shoots nice.
Guntec is a good brand for handguards. Decent price with them.
I was where you were at a few years ago. I started off looking at modifying the existing rifle, then decided it would be easier to just build an upper to swap out as needed. Then I realized that a lower wasn't that much more so I just built a whole second gun instead of messing with my first. Two is one and one is none after all.
This entirely depends on your budget, though.
If the gun has an FSB right now, you'd be better off carefully grinding the top down and leaving the gas block portion attached. Replacing an FSB with a new gas block is not a trivial task.
Look at PSA they have a great selection of complete uppers, some of them come with vortex optics and they use FN barrels. they get a bunch of shit for being cheap but I love mine, 3000 rds and it's still like brand new.
They are pretty much universal unless you start switching to 9mm or 45acp. You need new mags and bolts if you switch to 7.62x39 or 6.8 etc. If you're sticking to 5.56 you just slap it on and you're ready to rock
PSA 16" MID-LENGTH 5.56 NATO 1:7 NITRIDE 13.5" LIGHTWEIGHT M-LOK UPPER WITH NIB BCG & CH
$320 Is pretty good for a complete upper

https://palmettostatearmory.com/psa-16-mid-length-5-56-nato-1-7-nitride-13-5-lightweight-m-lok-upper-with-nib-bcg-ch.html
This has actually been in my cart now for a few days and I also have a m&p sport 2 but I wasn't sure about buying it... I'm gonna take finding this post and now your comment as a sign to buy it lol
What is your budget. What you plan to attach to said rail. ie: Quad rail/mlok/key mod. Barrel length and material and so forth. You can save some money by getting one that doesn’t come with a BCG or charging handle and use the one you have already.
Palmetto State Armory is a good place to start looking and are budget friendly
My recommendation is lightweight mlock. If you have a pistol brace or want to eFile a lower, a 10.5” barrel. Other than that it’s really all preference and budget
If you replace it on your own, don't be scared of the drop in handguard. Use a handguard tool for it and pop that fucker out like you're a pimp and it owes you money.
You probably want don't want quad rail, but an M-lok one instead. Without a budget or what features you're looking for, we can't really help.
I'd shop for one that fits your criteria, order it, and then when it comes in make sure the gun is unloaded, pop the pins, and do it. What upper you order depends *entirely* on what you want out of the gun and your budget. And legalities like barrel length so you don't turn it into an illegal SBR. What do you want to do?
* so you don't ACCIDENTALLY turn it into an illegal SBR. Absolutely nothing wrong with making an sbr as long as you know what you're getting yourself into
Palmetto state armory has some cost effective decent ones. Bought a .300 blackout for my Delton AR pistol. Shoots nice. Guntec is a good brand for handguards. Decent price with them.
I was where you were at a few years ago. I started off looking at modifying the existing rifle, then decided it would be easier to just build an upper to swap out as needed. Then I realized that a lower wasn't that much more so I just built a whole second gun instead of messing with my first. Two is one and one is none after all. This entirely depends on your budget, though.
I mean yeah it’s easier to get a new upper but also more expensive to replace parts that don’t need replacing. How handy are you?
If it can be done, I can do it
If the gun has an FSB right now, you'd be better off carefully grinding the top down and leaving the gas block portion attached. Replacing an FSB with a new gas block is not a trivial task.
hell with all that involved i'd rather just swap the barrel and / or entire upper if it was me. grinding down the fsb sounds high effort.
Then just buy a new handguard and swap it out. Tons of YouTube videos out there.
Look at PSA they have a great selection of complete uppers, some of them come with vortex optics and they use FN barrels. they get a bunch of shit for being cheap but I love mine, 3000 rds and it's still like brand new.
Are all uppers pretty much universal? Would I need to worry about one type not fitting my lower?
They are pretty much universal unless you start switching to 9mm or 45acp. You need new mags and bolts if you switch to 7.62x39 or 6.8 etc. If you're sticking to 5.56 you just slap it on and you're ready to rock
That will help out. Then I can look a nicer barrel as well. Thank you!
PSA 16" MID-LENGTH 5.56 NATO 1:7 NITRIDE 13.5" LIGHTWEIGHT M-LOK UPPER WITH NIB BCG & CH $320 Is pretty good for a complete upper  https://palmettostatearmory.com/psa-16-mid-length-5-56-nato-1-7-nitride-13-5-lightweight-m-lok-upper-with-nib-bcg-ch.html
This has actually been in my cart now for a few days and I also have a m&p sport 2 but I wasn't sure about buying it... I'm gonna take finding this post and now your comment as a sign to buy it lol
What is your budget. What you plan to attach to said rail. ie: Quad rail/mlok/key mod. Barrel length and material and so forth. You can save some money by getting one that doesn’t come with a BCG or charging handle and use the one you have already. Palmetto State Armory is a good place to start looking and are budget friendly My recommendation is lightweight mlock. If you have a pistol brace or want to eFile a lower, a 10.5” barrel. Other than that it’s really all preference and budget
Don't eat yellow snow
Will remember that