Fence panel repair challenge with concrete post!

[ghart_uk]

Hi,
I have concrete posts and wood panels. The posts are in very good nick but the wood has rotted and is now falling off the posts. The design is 3 lateral posts of wood with smaller flat pieces nailed vertically to these. The problem I have is that the lateral pieces were put in place with concrete posts out. Now that the concrete posts are fixed I can't slide in the lateral pieces. My only option it would seem is to cut the lateral bits in half and then re-fix them once slotted into the concrete posts either side. Anyone got a better idea? or how to re-fix the two pieces of wood?
Thanks!