Good idea. You're not the first.
I wanted to do CPL written instead of PPL and CPL written after, but was greatly discouraged not to bring 'different' (although higher) written pass paper for the checkride. I did read through FARs and there's no clause of substitution I could find implicitly.
If you get it approved from FSDO or Chief Counsel then it'd do. Would save you 300 bucks. The paperwork sent out from DPE has the original pass paper and if used for more than one checkride, you'd either have to get 3 originals or have it all sent out together. Hmm.