I am not going to try it as I value my 5 years of data, but it should work as I have not read anywhere that it won't (otherwise why the warning notice). If you want to start from new data then you could remove your current install, including data folder, and reinstall and start your settings to your choice.
If changing it in the Settings don't work you could try changing the cumulus.ini file (with Cumulus shut down) and edit under [Station] Pressure Units=3 (I presume Pressure Units=1 is mb as that is how mine is set and displays; Units=2 is hPa; and Units=3 is inches but I could be wrong so don't bet your data on at it, and it may be better to ask Steve at the Cumulus forum.
Paul