will this be a problem and how do I do a major update through NPM?

this shouldn't be a problem - if you're converting to Functional Stacks make sure you set their IDs to match your old ones. Once you update to sst 1.0 before you deploy you can use

sst diff

to see what infrastructure changes will happen

do I have to manually npm uninstall and then npm install serverless?

no you can just run

sst update

I'll read into functional stacks but as of right now the stacks are defined in a class, I'm guessing new stacks are defined in a function? also do I need to migrate?

you don't need to switch them for 1.0, class stacks are still supported

ok

I'd update to 1.0 first then later you can decide if you want to use functional stacks