I don't think there would be any problems from that. If anything, the current situation is more questionable, since you depend on GPL code. However, keep in mind that if you are to use AGPL (as you should), you will need to make available the source code to the version of SchemeBBS that the users interact with. Secretly patching the code and then withholding the changes for an indefinite time, like you do now, would not be permitted. Although, technically, since you are the sole copyright owner at the moment, you could just re-license it to yourself in secret. Please don't do that.
Sadly the AGPL is mostly based on trust, see this thread for example: https://wirechan.org/g/res/206.html