I can't speak about kindegarten, but my son went to a Reggio school from when he was 9 months until he was about 27 months, and then we moved him into a Montessori school. I was very worried (OK, this is an understatement... I was a basket case. I looked for over six months before I found a school that I thought was a close second to his Reggio school, and I shed buckets of tears over the decision), but he adapted beautifully. I think the serenity of the Montessori classroom had a lot to do with that. It doesn't have the glorious spaces of a Reggio room, but it shares some of the characteristics for the younger children.
The one thing that you will probably miss when you change is the communication and documentation that goes along with Reggio, as well as the community feel. With Montessori I have found that I feel very much an outsider, and the school itself is quite segmented into the different classes. It's not that the teachers make me feel that way or anything, but Montessori is so very much about creating a "world" exclusively suited to the child that I feel they don't include the parents very well. That said, I feel like I could push the teachers on that but I have not because my child is clearly thriving.
I cannot think of any learning environment that is better for very young children than Reggio, particularly for AP familes that need full day daycare. I really, really, REALLY miss the emphasis on creating art though. Montessori does not stress this nearly so much as I would like. I wish I could have kept my son in Reggio through Kindegarten. If your child thrives on Reggio, I don't think the transition will be too difficult.
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