If you put in the recipe as written, jot down the calculated OG, FG, IBUs, etc. Then, you can delete the extract and add in 2-row to get back to the target gravity. At least, that's how I do it.
That'll give you the basics. The speciality grains (crystal, chocolate malt, black patent, etc) will stay the same, although as you get more experience you can start adding in specialty malts (the kind that need to be mashed, like Munich and Vienna) as appropriate. You also MAY need to make some minor adjustments to your hops if you are now doing full boils (less concentrated mash = higher utilization).
Personally, I think BeerSmith (also with a free trial) is an easier program to work with than ProMash, although both are very good. Just a personal preference.