I submitted those dates, which come from knowing staffers, possession of letterhead, and personal memory. I believe someone else submitted the start date of 1999 for Advantage, and I remember there was nothing between PYCCO and Advantage. They kept PYCCO for a lot longer than one might expect, but I am not 100% certain on 1999 - it could have been 1998.
April 1991 was the switch from an '80s set, that '70s TV set logo and "Move Closer To Your World" to "People You Can Count On." They kept PYCCO until they did a long overdue update of their set and graphics in the late '90s. The new logo included the text "People You Can Count On" even though they'd dumped the theme. Honestly, I would love to find proof and confirm the dates.