There is a bit of misundestanding… the question is related to “reversion”, not “congestion”.
When there is reversion to old channel, it means the BSC has already allocated the radio channel in the target cell. The reversion only happens when the MS tries to connect to the target TCH (after the HO CMD from BSC).
Therefore, if there is congestion, the MS will simply stay put in the serving cell : it will not even know there could have been a HO. I wouldn’t call that a “reversion”.