Please forgive me for entering this discussion at this stage, but I am neither a master of Inkscape development nor of SVG structure. So, I hesitated to make any comment because it would necessarily be only slightly informed by having worked with other data structures that also evolve as the programmers work on the programs that create and maintain them.
Specifically I have worked with the Initial Graphics Exchange Specification (IGES) and the Product Data Exchange Specification (PDES). Both of those standards attempt to move data from one computer-aided drawing/design system to another through the neutral data format specified in the standard. There are several small companies that work to customize SysA to Standard to SysB solutions. None of them ever attacks the entire file from SysA in one all encompassing translation or fix-up utility. The problem is too complex for such a solution. In fact, if you look at PDES, you'll find they carefully set up Application Profiles that identify workable subsets of possible engineering data and only work one of those subsets with each utility. Even then the subset may be too complex. All the programmers expect trained users to babysit the translations and report back errors in the translation. Some don't even release their translators, but hire themselves out to intelligently use the tools, because they know they are the only ones who know what the programs can really do accurately.
To sum up, my suggestion is you may need to attack the problem of evolving data structures from a more granular perspective than 0.47 to 0.48.