Seems to me, though it's none of my business, that there's a little "apple to oranges" and "assuming outcomes before they happen" going on here.....
The WN example of not being able to renegotiate a CBA after changing representation is accurate, but it's also apples to oranges. WN was (& is) a single carrier with a single group of mechanic/related that had a valid contract that wasn't amendable. None of those factors are applicable in the current IBT/IAM situation.
Likewise, assuming that the IAM will be successful in becoming the representative of the mechanic/related for the merged company is assuming events that haven't happened yet. If it happens, the IAM contract will survive and isn't amendable till 2009. Any transition agreement will merely spell out how former IBT members will transition to the IAM contract.
There is, of course, another possibility - that IBT is selected as the representative of the mechanic/related for the combined company. Since the IBT contract is apparently currently amendable, Section 6 negotiations would continue (assuming they hadn't been concluded by that point) and it is possible that some success could be achieved in combining the best of both the IAM & current IBT contracts, though nothing is guaranteed.
So it seems to this outsider relatively simple. If you like the IAM contract, support the IAM - you'll have that contract through at least 2009.
If you like the IBT contract or want a chance to improve either contract, support the IBT - you'll have a chance at making changes through current IBT Section 6 negotiations (again, nothing guaranteed).
Jim