That's a great question, and one that everyone needs to consider when moving from one website platform to another.
There are a few factors:
- How good the current site is
- How good the new platform is
- How the transition is handled
Since 70% or so of your SEO ranking comes from off-site factors, it can be tempting to overlook the importance of transitioning well. (see below)
If the old site is junk for SEO, then moving to another platform should help.
If the new site is junk for SEO (flash, frames, etc) then not much will help you.
In your case the TF sites are now approaching OK for SEO, so you want to make sure that you don't lose what's been established so far. The biggest part of this will be implementing 301 redirects properly and quickly.
A 301 redirect is a server message that tells the web browser and the search engine that the page that formerly lived at this address has permanently moved to that address. It's like those telephone company messages that say "The number you have dialed has been permanently changed to..."
You don't need to know how to do 301 redirects, you just need to know that they are being done for every page on the old site. As much as possible redirect to like pages (Christmas -> Christmas, About -> About, etc), not just to the new home page.
Two benefits of a 301 redirect:
- Tells the search engine to attribute all links, authority and social credit from the old page to the new one
- Seamlessly moves customers to the correct page if they followed a link or bookmark to the old page
You really want to have search engines find the 301s before they find the new pages, otherwise they will treat the new pages as new - no authority, no track record, etc.