Armor decision

Wearing mithril plate , just found ommel tooth plate, should make the switch?

Sure, why not?
Make sure you keep that mithril plate around for when it starts to rot though. If possible you should cast Slow on the plate as much as you can.

As far as I can tell the slowed status on an item is permanent and doesn't stack, so it should be necessary to slow the ommel tooth plate more than once (unless someone casts haste on you).
How's your leg strength, and how much do you like your agi+dex scores? I'm going to include adamant (which is easily accessible through loricatus+food) in the following comparison:
Mithril plate armour reduces agi+dex by 10%, ommel tooth and adamant by 20%.
Mithril and ommel tooth plate armour together weigh the same as adamant.
Mithril provides just over 50% of the AV of Ommel tooth, which has about 7% more AV than adamant.
In other words, wearing ommel tooth and carrying mithril around as a backup is more or less equivalent to just wearing adamant, with the added hassle of worrying about when it'll spoil.
So the question boils down to: would you swap your mithril plate for adamantine plate? If so, then switch to the ommel tooth (and try to turn a spare plate armour into adamant before it spoils).