That's awesome that you have the ability to upgrade an interface to Thunderbolt. I'm stuck with the MOTU 828 MK3 FireWire only interface, not even the Hybrid unit that came out shortly after I got the FW only unit.However, if you at least have a desktop, you can install a Texas Instruments-based PCIe FireWire 400 adapter card. I've had mine since 2014, and works great! As long as you have an open PCIe slot, you should be able to add one.Now if you're on a laptop, that's a different story.
I do want to let me know about an experience I had a few months back that I was not aware of in regards to FireWire 400 6 pin interfaces though. While 'technically' it's hot pluggable, a FireWire 400 6 pin to 6 pin cable I had sitting in my drawer fried my interface during hot swapping. During a normal troubleshooting session, I managed to fry both ports on the 828Mk3. Fortunately, MOTU offers a program to fix your old MK3 still for I believe $100 + shipping. Now I'm not 100% sure thinking about it that it was the hot swap that did it, and if I had it powered down, did the cable swap, and powered it back on that it would have been fine. However, it has made me think twice about using unknown cables in my drawer for FireWire. Because the 6 pin variety does carry voltage (~30v on pin 1) it's within the realm of possibilities that the hot plug killed it. I'll never know for sure (old cable immediately went in the trash and no way willing to risk my interface again
) but I didn't think about a cable frying my main interface.