It seems you should be able to simply resave RNC format to RHP format in Encore.
Convert Encore musical notation file format to Encore file format file.
We found 2 software records in the database related to .enc to .rhp file format conversion.
Microsoft Windows software - convert enc to rhp on Windows
A notation score writer application
Convert from enc file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned enc to rhp. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert enc to rhp on OS X
A notation scores writer and music composing software for Mac