With that cost, work, and protential damage to the iMac. I personally think that's not quite worth to upgrade the CPU for "just in case". The difference between i5 and i7 basically just the multi-core speed. There is not much software that only able to run under i7 but not i5. So, if that "special moment" come, most likely your i5 can handle the job well, just a bit slower.
Of course, it will be another story if you plan to upgrade to a significantly higher clock frequency CPU.