A provider I saw a few days ago asked me to write her an updated review because she hasn't been getting many since she has been traveling alot.  

It's been over a year since the last time I've seen her so I don't mind writing one.  I gave her two 10's last time.  Is this common?  Does my old review just get deleted?

I saw her 3 days ago.  My last review for her was a year ago.  Im pretty sure 3 days is within the 3 months.

New review will replace the old one I think

I believe this is all spelled out in the Review FAQ.

was in something other than an OTC situation since your final sentence says "It's been over a year since the last time I've seen her..."

It won't be readable under your profile...only to you under "your reviews" under "My TER".  New reviews supplant any old reviews as far as the outside world goes...

"Your new review must be a complete rewrite that adds something new to the experience in order for it to replace the old one."

So as in my case as I went from first time experience review to an ATF review a lot had changed and I described it, but you just can't review again to show she is still active.