In practical terms, I think you have no worries until the distant future.
While 64-bit applications are supposed to be the big and coming thing, there are very few of them yet, and I have seen none that show any great advantage over their 32-bit counterparts. (With the exception of overcoming purely Windows 32-bit limitations like memory usage).
The day when all support for 32-bit applications is completely removed, is very far off. Your existing programs will continue to run very well for the next 5 years beyond doubt. (Shoot, there is a whole host of ActiveX controls, COM objects, VBA add-ins and whatnot that are still 32-bit with no current plans to convert them. They aren't going away anytime soon. Even MSOffice had to acknowledge that.)
Somewhere in the next ten years, you may need virtualization to continue to run them. Here, I'm looking at things like the complete disappearance (for all practical purposes) of the parallel and serial ports in favor of USB and firewire -- that kind of general system change that creates problems which your existing programs may or may not handle gracefully.
Your horizon is somewhere between 5 and 15 years, then. A bigger problem will be finding anyone (then) who can still program in Foxpro, because in that time frame, the bigger threat is that your business rules will change and things will need to be re-written. Not many businesses survive on the same rules for that long.
When that happens, at some point it will not be possible to work around the software anymore, and you will be forced to change.
Whatever platform you might change to now, no matter what that might be, has a good possibility of being obsolete by the time that forthcoming Day of Wrath rolls around, and if it is, you would have to change your platform again.
You know you will face one Dies Irae out there, someday. Creating an artificial one now will just double the expense,time and hassle with no real assurance that it will make that future transition any easier, faster or less expensive.
While I'm all in favor of keeping up, technically and technologically, you've got a large investment in your existing software, that isn't in any near-term danger. Start changing your current and future development, sure, but that's a different matter.
