Converting a bex query/report from 3.X format to 7.0 format is simple and easy. You just need to open 3.5 query in 7.0 designer and save it. The query will be saved as a 7.0 query. One thing to keep in mind is you can no longer edit this query using 3.X query designer.
Reasons to/not to Convert 3.X query to 7.0:
Bex 7.0 suite has some nice features as outlined in the article Bex 7.0 features. We can still access BW 3.5 Bex queries from 7.0 bex analyzer. So it is not necessary to convert query to 7.0 format to be able to acces them in Bex 7.0 Analyzer with new features.
In my opinion one should only take the effort to convert existing queries when there are changes or if you want to use some of new features in 7.0 query designer.
