I'm surprised at all the recommendations to take the upgrade on a different aircraft. Is the OP not correct in being cautious as he is being asked to 1) learn a new aircraft and 2) immediately take full responsibility when in the cockpit? If given an upgrade in a type he's been flying for years, I'd see how he'd be stupid not to accept but new type and LHS straight away? Is that not that big of a step as I imagine it to be?
I expected more recommendations of waiting out the 7 months and be a better captain in an aircraft he knows very well.