1. If you give one vendor an opportunity to change their proposal, do you have to give them all an opportunity?
2. Does it matter if you are using simplified acquisitions procedures, GSA or GWAC contract?
There is a GWAC guide you might find very helpful. The GWAC ordering Guide of 14 April 2015 states, OCOs should evaluate proposals based on the methodology stated in the solicitation to maintain fairness in the Order process and mitigate protest risk. Either tradeoff or lowest price - technically acceptable evaluations are valid best value methods, and are authorized at the Order level.
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In your situation you stated , "The basis for award states price will be the determining factor for those offerors determined technically acceptable." The problem now is the fact no one was technically acceptable and you want to do something different than what you told the vendors you would do. My recommendation is to hold discussion with everyone and request a revision and make award to the lowest priced technically acceptable , as you told the vendors you would do.
This answer is based on the information provided and you may want to consult with legal.