I tried that a few years go. It worked fine, until at got to a certain percent or so (not sure what percent that was or if that has anything to do with anything), but basically, the drive died and I ended up with 4 Flashing Lights. I made a topic on it awhile ago. At the time, I was still new to TiVo so I was still on a, I think it was a 90 day service trial. My TiVo died before that service ended, and I ended up just buying another Bolt, but actually it was the Bolt+. So, the new service, they put on my new device.
There was one user that had two Bolts and it magically worked on at least one of his devices, had it working on both but eventually he said one of them got the 4 flashing lights.
I don't know exactly what causes the 4 flashing lights, whether if it's using a drive that is larger than what a 32-bit OS can address, or if it's something to do with the drive type. I've been told it's because of the drive type, for the most part. SMR vs PMR, if I remember correctly. Whichever TiVo's is, that's what's hard to come by.
Just throwing this out there, but I have zero experience with the new Edge device. I've heard the 6 tuner version only comes with a 2 TB HD. But I've also heard that the whole system in general is more forgiving for hard drive upgrades.
Anyways, I wouldn't bother upgrading. Again, I tried multiple solutions, replacing the hard drive, and then also I tried purchasing a SATA cable that would feed out of the box to an External HDD Enclosure. With both solutions, I still ended up with the 4 flashing lights. So, I wouldn't bother. See if TiVo runs a service transfer special to another device and wait for that.