Boot Loader Issue

[Doug Hardie]

I had to convert a system from GPT to MBR.  All went fine till I tried to reboot the system.  It gets to mountroot and dies trying to mount from ufs:/dev/ada0p2.  That won't work.  If I enter ufs:/dev/ada0s1a then the system boots fine and runs.  I need to alter mountroot so it tries the right partition/slice.  How do I do that?  I couldn't find anything in the handbook on that.