I just went through a similar problem trying to change crystal frequencies.
This is what I found:

DEFINE is only available in PicBasic PRO.

In MicroCode Studio, you can change the oscillator type, XT, HS, R/C, etc.
After you have clicked on "compile and program", the next box to come up is "meProg-......". Click on VIEW and then CONFIGURATION ( you might have to click CONFIGURATION twice). The meProgram screen will come up and you can make changes to the oscillator there.

You will still have to compensate for a different oscillator frequency in your program.

Good Luck
Brewster............another newby