I've translated a pretty experienced character from SR3 to SR4 (400 BP). He is really really weaker (and underpowered compared to a character normally built, without trying to respect some caracteristics of the old one), but I just gave an explanation in his story why he has lost so much of his old abilities. So yes, he is weaker, so what ? he's much more fun to play in this version than he was in the SR3 one, even, and maybe because, he's weaker, old, angry and can't help but criticize the new runners' bach.
If you have to translate him into 400BP, he will be weaker. Try to find why your character is weaker than he was, then you'll find what you have to sacrify in his old abilities. Maybe he had a cancer, and doctors had to pull out his chrome in order to heal him ? maybe he fell into coma during the crash, and wake up with a less sharp mind ? maybe he became addict to a dirty drug, and hardly survived, his body still have sequels of it ?