Generally speaking, you would never acquire enough condensation in a gas line to be problematic. If you're having moisture issues with a gas line it's almost always ground water entering the pipes.
Shelly I think the "go for broke" plan is best. The newer lines are going to have a lot longer longevity, and you'd need to replace all the old eventually in any case.
It's also a good idea to draw a little map of the new lines for any future digging. Even though you always want to call for locates before digging (to save your own ass), I can't tell you how often the gas company, water company, cable company, etc... get's it wrong on their locates.