This means removing ABBC 3.1 will not break your old posts, for the most part.
All of ABBC 3.1's BBCodes will still be present as regular custom BBCodes. You can edit and/or delete them if you choose by visiting the Custom BBCodes section of the ACP.
There are only a few special BBCodes, however, that will not work as expected if ABBC 3.1 is removed:
[BBvideo]BBvideo (will revert to a url link)
[fade]Fade text (will no longer fade in/out)
[hidden]Hide content from unregistered guests (content will be hidden to all users)
[spoil]Spoiler message (Spoiler content will be hidden)
[youtube]Youtube legacy BBCode (will revert to a url link)
You can re-install ABBC 3.1 at any time and all its BBCodes will be restored (even if you edited or deleted some of them).