Would changing a React.js project to Next.js require many changes to the code?

I have a project that is due in a very short amount of time. Preferably, it would be built with Next for better SEO, but currently I don't work with Next and I am worried that going down that road would take more of my time than I predict.

I'm wondering then what if I'll do it with React.js just with a create-react-app which is what I've been doing so far, and if I have time left I'll jut change it to Next.js. Would that makes sense, or would that switch require too many changes?

** I use Firebase as my back end. I feel like that requires even more adjustments no? As it's server side rendering? Would love any opinions on which direction you think I should take.