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This is a bit of a thorny question, as Wizards of the Coast seems to forget that when new rules mix with old,they need explanations. Normal mounts aren't an issue, as they have no "advancement" entry. The griffon, however, may be tricky. While it can go either way, based on DM ruling, the rules as they are written say yes, it would increase in size. This, however, may make it overpowered (I've never used one, so I wouldn't know) so if this happens, take your player aside and politely tell him it needs to go back to original size, for game balance's sake.
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