Honestly go for it. It's your game. The fact that the game was free this long is freaking amazing and like you said yourselves, it becomes free again in the future. So if ppl can't pay, there are enough games that are free that they can play till this one here becomes free again. Games are a luxury, like art is etc. so ppl need to decide for themselves if they are willing to pay. If not then obviously it wasn't worth enough for them and then that means that you don't need to play the game that desperately. Tbh I personally didn't see the price change and only now your post about it, so for myself it wasn't really a big deal either but yeah I think for the future it does definitely make sense to write the post beforehand, just for the transparency of it and as a heads up. But other than that, ofc it's your decision :)
Wish the best of luck and success for you two and the new game!!