I'm feel so cross on your behalf, mainly because of the lack of consultation. If the class is going to be run properly as a mixed age class, with specific consideration made for the Reception children, then it might not be all bad, but if they entend to simply slot these Reception children into an established Y1 class, treating them as accelarated Reception children following a Y1 curriculum, then I'd be livid. It's such an important year, and hugely important for all Reception aged children, whatever their academic ability.
Possible questions to ask are...
Will the children still be following the Foundation Stage curriculum, with the teacher planning learning experiences from the Early Learning Goals (specifically designed for Reception aged children, and a statutory requirement and right).
Will their learning still be planned from each of the six Foundation Stage statutory areas (Personal, emotional and social, Creative development etc) with equal status given to each area, or will they be following National Curriculum schemes of work (History, Geography, Science etc designed for Y1)
Will they still have access to learning through play, with sand/water/role play etc available to them, and learning in an outdoor environment? Is the Y1 classroom properly resources with these things, and appropriate Reception equipment (construction stuff, puppets, puzzles etc)
How do the school plan to cover things like phonics and numeracy teaching, where the YR and Y1 children will be at different stages?
If it really is a 'done deal' then you need to make sure they are doing it properly. I hope you get some reassurance soon.