I doubt there is a simpler solution, and I am also mystified as to why you want to do all this work in the first place. There's no problem at all with long file names, unless you bump up against the operating system limit for characters in the path/name.
By the way, writing metadata to XMP only writes some of the catalog information to XMP files. There are items in the Catalog that are never written to XMP, and cannot be transferred this way. A partial list:
- collection membership
- edit history
- pick flags
- virtual copies
I am also mystified as to why you want to do all this work in the first place
Its not that they are too long, it is that there are character/numbers at the beginning which are messing up my naming scheme. I like to keep my file names consistent!
Good to know about the XMP metadata.
Without knowing the reasons behind your file names, which are important to this discussion, let me say that the whole idea of organizing by file naming schemes is not the way Lightroom was designed — yes, you could continue to use Lightroom with that naming scheme inside of Lightroom, but as you have noticed, there's not an easy way to do this. The better scheme of organizing inside of Lightroom is to use keywords and other metadata as your organizing/search scheme rather than using file names.
Also, since I don't know how much work you have done in Lightroom so far, you could extract the photos from Aperture again and import the photos into a new LR catalog (which cause you to lose edits and other user-supplied metadata, definitely a drawback) without performing the rename. I suppose it's a tradeoff between the advantages and disadvantages of each path forward.
If the characters at the head of the filenames are identical, you could use the Search Replace Transfer plugin to remove them: Complex file renaming in Lightroom made simple – thanks to the Search and Replace plug-in – Lightroom Solutions