You can't change them, but you can create a new sequence and Copy/Paste everything into it. Just make sure to expand the sequence to frame level and check all your edit points afterwards.
Tried that, but seem to have lost all video transitions, any way of keeping them?
They normally don't go anywhere. Just hit CTRL-A in the old sequence to select everything, CTRL-C to copy, then move to the new sequence and hit CTRL-V to paste everything.
Yeah, that will happen, because the timebase is different and the edit points will all have shifted--in fact, they'll often shift to the "middle" of frames, which aren't legal time demarcations. You'll end up with a big mess when you try to export.
First, tweak all your edit points; you can zoom in and then just drag the ends. They'll snap to the whole frame demarcations. Then, drag your original timeline panel out into its own panel; if you grab the sequence tab and drag down, you'll see a purple trapezoid appear and you can drop the panel so you have two visible sequences. From there, you can copy from the first and paste into the second--it won't be fun.
If you're using a standard transition and duration, you could also select all the clips in the new sequence and go to Sequence > Apply Default Transitions to Selection.