it would be easier to remove the seal if the cover was off but it may cause a few problems when trying to make sure it seals up properly when reinstalling,as you also have to remove some fwd sump bolts aswell,
the only difficulty is removing to old seal.and once its out its very easy to install the new one,and with less work involved than putting on the front cover.
i would try this way first and if all else fails you can always go the other way