Exactly, it can't be worse, so give it a shot. You could try mixing it with the current formula to ease him to the new flavour... do it over 4 days... 1/4 HAF with 3/4 Gentlease the first day and then 1/2 & 1/2, then 3/4 & 1/4 then the 4th day would be 100% HAF in the bottle... This can help LO's take a new taste without as much refusal...
As for the constipation issues - do you give him water during the day as well? Not really needed when bubs are BF'd but FF babies do need extra water, especially with the addition of solids. What solids are you offering now? Maybe something will jump out as the constipation causer...