BeerSmith [2.x especially] has a 'Convert' button that will let you take a recipe from/to extract, partial mash, or all grain... It even lets you select what to use for a base grain when converting to all grain. While the base malt conversion number is nice, it doesn't paint the entire picture. There are other things that you'll want to adjust too. Hop bills for one thing. You'll also want to figure out your efficiency (over a few/several batches) so that you can get a better idea as to how much base malt you need to use to get the same OG...