User blog comment:Xilinoc/Ch. 553 - Frozen Cross/@comment-5695542-20131017021841/@comment-10630156-20131017224651

my guess in this matter is that Ichigo's real Zanpakuto is a Hollow as well and as we know Hollows are poison to Quincies so the powers of the "old man" is weakened when he releases Bankai due to his connection with the real Zangetsu becoming stronger and so his appearance changes to looking younger.