What you are planning is 100% OK, and very common! Many commercial homebrew games started life as NESDev Compo games ( Twin Dragons, Nebs 'n Debs, Project Blue). My own game, Witch n' Wiz, did exactly what you are planning: used the compo as a way to build a demo and an audience, with a planned eventual commercial release later that year.
The rule is more about ensuring that every entry is "new".