It's difficult to change without removing the crank, but can be done. Twice I've been in the process of rebuilding an FE when I forgot to install the rear seal before installing the crank. I tried to slip the seal-half in there, but the crank had to come out and this was with the block upside down on an engine stand. Also, those side-seals can be fiddly. I wouldn't be comfortable torquing the rear bearing cap while under the car either. So... leave it if it isn't leaking. A tiny leak (as in no drips, but a little damp with
oil)? Leave it, or take the engine out.