Nobody can know. We do not know which tutorial you are referring to, not your version of AE, no specific project details, not the code of the expressions you may be using, not your keyframe timing.
I can only guess what you are talking about. If you tie transformations in a main comp to layers in a pre-comp with expressions changing the length of the comp or the pre-comp will have no effect on the expressions. If you lengthen the length of a composition then you'll have to extend the layers inside that comp because the layers will not automatically adjust the out points. You'll end up with no layers beyond the original length of the composition.
Also, you said that this is stopping you because you will have to overwrite your footage. This doesn't happen because After Effects is NON Destructive to your footage. When you render you make a new video and the original is not changed. There is no possible way to overwrite your footage in AE.
If you simply want to add a title to your video then why not create a new comp that is long enough to add the title, nest your main comp in the new comp, move it down the timeline far enough to leave room for the title, and then add your title.
Sorry, I just don't understand the problem. Maybe you could explain exactly what it is that you are trying to do or provide a link to the tutorial that is giving you problems.
Rick, although I won't know until later, it sounds like the fact I've been lengthening the comps and not the layers may well be the issue.
Apologies for not including the video, but as it seems I'm allowed to do so, here's the video that includes the linking Rotation, Position, and Anchor points... VIDEO COPILOT | After Effects Tutorials, Plug-ins and Stock Footage for Post Production Professionals
Also, thanks for the idea of making a new comp just for the title -- great idea!
I'm still a bit new to this, so I apologise if I'm asking silly things -- I'll get there!
Thanks for responding Mylenium -- fair points; I'll bear those in mind for my next inevitable query.
The link I posted in response to Rick's contribution contains the exact work I've done (linking two comps to counter rotation etc.), but I think I know where to go from here.