Hi.
What is the current firmware version on the modem?
Which firmware version are you going to downgrade?

my modem hardware version is v3.1 and i accidently upgraded it by v4 hardware firmware ...
at now my modem firmware version is TD-8817_V4_100524 but before upgrade it was TD-8817_v3_090211 ...
my modem working good but I afraid cause some damage to it .
I tried to downgrade it to original firmware but when select original ras file show me this error. :: fail to update due to ... the uploaded file was not accepted by the router .

Since your modem works great with Firmware 100524, I think firmware for V4 is also suitable for V3 on TD-8817. And firmware 100524 is newer than 090211. So there is no need to downgrade it to original firmware.

it's really weird you can upgrade a v4 firmware while you use a v3 device....
but if it works ok now,just stay the same,you needn't upgrade it to v3 firmware