It can be hit or miss. Of the last four mobo swaps I've done in the last couple of months, I've had to reinstall twice. Most people take for granted that a reinstall is going to be necessary. I haven't run into this issue, but I've read that sometimes even if it will boot into Windows you can run into software issues.
you should just do a reinstall since like said it can cause issues in the long run. Plus you need to delete the previous mobo drivers or anything else you replace.
I think it depends on how closely the mobos are related. The swaps I did without fresh install similar to what chromenut is contemplating, namely going from a Hero VI to a VII. I changed out many components at the same time (CPU, RAM, some of the drives), but I didn't have any issues. Same thing for a Gene swap. The other two were changing brands and socket families.
i think it will conflict since the 970 needs a bios update while the 990fx doesnt which means something needs changing i would just back up the things i need, get a ssd if you dont have one, and do a fresh install.