I'd keep most of Boosters' history but Extreme Justice didn't happen. Booster just got armor until Superman helped him replace his costume and he never lost an arm. ICBINTJL/FKATJL still never happened. He also has his golden pants.
Marriages of Clark Kent/Lois Lane, Barry Allen/Iris West and any other current unwed couple that were formerly husband and wife still exists (except Ollie/Dinah.) Superman is not a brooding loner as that's Bruces' job. The Kents are alive and well with Chris Kent in limbo awaiting their return.
Barbara Gordon is still Oracle and while a little bit younger is still Dick Graysons' senior. Cassandra Cain and Stephanie Brown fight crime together in the buddy team up book. Cass was never Tims' villain, she never worked for the league of assassins.
The WTF Jason Todd stories (a.k.a. the blob from Nightwing, BFTC, the red head stories from Morrison/Winick, and Countdown never happened. Countdown happened for no one.) The only in canon stories for him as Red Hood are all the non-red headed Winick penned stories and Johns Teen Titans appearance. Since Jason is younger now (canonically he's almost 2 years older than Tim) the thing with Talia has to be taken out. He was barely legal as it was.
Connor Hawke and Mia both exist. Lian (along with other babies) is alive even if she's a bit younger. Roy had her when he was what, 18/19? If need be he could have been younger. She's the reason his character had such a huge impact on me. A single dad that's a super hero and recovered drug addict. I don't care if this makes Ollie a grandpa, he wasn't around her that much anyway.