Mmmm.
I’ve no experience on this, but you could consider software which encrypts / decrypts on-the-fly (by-the-way, there is also a nice section on the “why” to do it, to go back to the initial question):
Data-at-rest encryption - ArchWiki?
But then for Full-disk encryption (FDE), see dm-crypt/Encrypting an entire system.
And here, if you search a little bit also on the internet, you’ll see that almost everybody is discouraging the possibility to encrypt after installation, moving your data back and forth or chrooting and doing whatever. They always say that reinstalling is faster and more secure…