I think it is okay as long as the older work you are doing is also done in Krita from scratch. The rule says it needs to be done in krita from scratch it doesn’t say when. Although some time back we had this discussion about submitting older artwork done in krita. Some said it is not fair since other people need to do in the required time frame while the submitter may have done in leisure. But in your case I think it is okay since are you only using part of the artwork. Probably like using textures pre made in krita.