We have a document revision tracing system that tracks the latest version of hundred of documents. We had previously used:

=IF(SUMPRODUCT(--(G24:BD24<>""))=0," ",IF(MAX(G24:BD24)=0, LOOKUP(2,1/(COUNTIF(G24:BD24, ">" &G24:BD24&"~")=0),G24:BD24), MAX(G24:BD24)))
to find the highest letter or number and display that in a summary column. We use alphabetical revisons for pre-production and then switch to numeric revisions once in production.

Now we have documents going past "Z" revision and so are using "AA", "AB", "AC", etc. but the formula above won't work anymore because "AA" sorts to between "A" and "B" and not after "Z".

Any suggestions?