Are L-shaped and T-shaped Magsafe 1 interchangeable?
In short no, there's no problem.

Apple changed the "old" T-models with new L-models.

However, they are exactly the same and I've been using a 85Watt MagSafe charger for years now as it charges much faster.
Just do it, and nothing will change, except if you go from 45/60 watt to 85 (it will load faster :)). It's just a new look, the connectors are exactly the same.