Oracle817 on 2.8 should recognize all the oracle files (init.ora,controlfiles,redo log files,data files) from 2.6 if the OS is configured similar to 2.6 (32 -bit not 64-bit) and filesystem layout is same as before(If not you might need to recreate the controlfile). So, cold backup would be sufficient.

You can test this, if you want by recreating the controlfile with system,rbs,redologs,init.ora and couple of datafiles(not all of the datafiles).