11ac is backward compatible with 802.11n. Most of the modulation are same but 256-QAM is not supported in 802.11n (lets forget turbo-QAM for now). Normally is there any problem when a radio has to change the modulation for different clients ? since a radio at one point can only use 1 modulation (assuming single spatial stream), when it communicates with another client with different modulation, will it impact the performance ?
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I doubt there is any significant delay due to such a transition.
If you would like to look into the subject to a deeper level, I would recommend Mathew Gast's Book
802.11 Wireless Networks: The Definitive Guide.
My understanding is that when two 802.11ac STAs communicate they'll use the corresponding PHY layer, ie. VHT mode. The 256QAM capability isn't the only thing that differentiates it from HT mode (ref. the PHY chapters in Matthew Gast's other books, ie. the survival guides on 802.11n and 802.11ac) Try googling "VHT-SIG-A vs HT-SIG".
When you mention different modulations, do you mean sequentially or in parallell (wrt. time)? If you consider an AP communicating with multiple clients in parallell, it's called MU-MIMO, which is .11ac-only.