Are you looking to switch the 2 tanks and asking advice on how to do that?
Do you have the proper equipment yet to run the 72 as a SW tank..a skimmer that is appropriate for that sized tank, new lighting, enough live rock so that a cycle would be minimal, enough space to store all of the new SW you will need to add... etc?
Since it was FW, have you run copper in the FW tank before? might be an issue.
If you already have a SW tank, you should already know what you need to make the 70g a SW tank also. The rest is just moving rock and livestock over and adding fresh SW. Just remember that the sand and any new live rock will cause a min cycle. Nothing probably to worry about but expect it.
It will be far easier if one of the tanks is empty and you aren't switching the 2 of them. Better yet if you can cycle the 72 before you move your SW fish/corals onto it.
Perhaps you can trade your cichlids back to the LFS and get credit for items you will need for the larger SW tank?
Are you planning to use a sump with the 72? Are you going to drill it or use an overflow box?
..and as Tosiek asked, has the 72 ever had copper in it as a treatment for the FW fish?
Upgrading 1 tank is a huge task, trying to do 2 at the same time will be near overwhelming to keep all of the creatures safe during the switch, especially if you have to add a sump and plumbing to the 72 to turn it into a reef tank.
You'd need totes, air & heaters for all of the fish, and another tote for the SW itmes and corals (if you have them), along with circulation for that tank. As I mentioned earlier you will also have to have all of the water made upo for the SW tank before you begin, so that is more totes and buckets.