What firmware revision is the 1756-L61 using?...
Just one consideration that may or may not be relevant here...
A 1756-L61/A processor supports firmware revision r12.xx to r20.xx.
A 1756-L61/B processor supports firmware revision r13.40 to r20.xx.
A 1756-L72 processor supports firmware revision r19.13 or later.
If the 1756-L61 processor is currently using firmware revision r19.13 or r20.xx, then the project conversion to a 1756-L72 processor will be relatively straight forward as the necessary version of RSLogix 5000 is already installed.
However, if the 1756-L61 processor is currently using a firmware revision previous to r19.13, say r17.xx, and you do not have RSLogix 5000 v19.xx or later installed, then you would have to install the necessary version before performing the "Change Controller" procedure i.e. RSLogix 5000 v19.xx minimum.
Most contract programmers have most to all these versions of RSLogix 5000 installed, up to v20.xx and beyond, but some company based programmers may only have the highest version required to support the installed base. When I arrived at my current company in 2010, the latest they had installed was RSLogix 5000 v13, as at the time that was the latest they needed. I quickly rectified that. I currently have v10 through v28 installed and at my disposal, whether I need them, or not. I suppose it's hard to let the contractor in me go? He is still quite useful now and again.
Another consideration here is if you wanted to go "new" with your L72 firmware and flash it to r21.xx or later. These revisions, of course, require Studio 5000 and Logix Designer software. If, again, you do not already have the necessary version of Logix Designer installed on your workstation to do this, you would be taking a leap from the v20 and older era to the v21 and newer era of programming software. This, in itself, can introduce headaches with compatibility issues between the newer and older versions of software you would have installed on your workstation.
If you're "fully covered" for this newer controller, software-wise, then ignore my observations. If not for you, it might prove to be useful info for others.
Regards,
George