macOS Catalina runs only on 64-bit, so when I reload my existing Microsoft Office files, will they be "read only"?
The 32-bit/64-bit description refers to executable code, rather than data. Applications and other executable code will contain instructions that are either 32 or 64 bits wide.
However, file formats are largely unaffected by this. Almost all applications will open the same file data regardless of the application's bit-depth. So, MS Office documents can be opened by both 32 and 64-bit versions.
It's possible that new software may introduce new data structures than are unreadable by older software, but that's a different issue.
No. Macs have been, hardware-wise, 64-bit for a long time and even if Mojave technically supports 32-bit apps, most recent apps including Microsoft Office are 64-bit.
Even if it wasn't, things like text files and word files and spreadsheets and etc. aren't specific to 32-bit or 64-bit systems. Editing a file on a 32-bit system won't make it "read only" on a 64-bit system. Even if you were upgrading your system from 32-bit to 64-bit, your documents would remain the same.
Perhaps if you were weird and using some 32-bit office suite, you would have to upgrade the suite itself in order to start the program that can read those files, but the files themselves will be fine.
32-bit and 64-bit only refers to executable programs, specifically the type of instructions the CPU understands. Anything that is not machine code does not care about 32-bit or 64-bit - that includes documents.
There is a very good Apple suite of tools that are compatible. It used to be called iWork but now it just each individual apps. They are Pages, Numbers, and Keynote. These can replace Word, Excel and Powerpoint and read / write those formats.
If you’re worried any third party app won’t work with the new OS - try the alternatives so you know how you like them.