I have a few suggestions, based on sending some AACR2 records through your conversion process:

A lot of "AACR2" records these days are actually hybrids: formerly pure AACR2 records that have had some RDA elements added to them (this is also happening in OCLC). You might check if elements already exist before adding them to the records, or check if a record is already marked RDA in the 040 before making any changes to it, since a lot of collections are mixed.
Subfield order is weird for 040: ‡a, ‡b, ‡e, ‡c, ‡d, so when you're adding ‡e rda, it needs to go before ‡c and ‡d
Some ISBD/catalog card silliness: If the 300 field ends in "cm." (or "mm." or some symbol that doesn't have a period inherently part of it like "in." does), that period should be removed unless there is a 490 in the record.
Quoted notes in 500 fields used to list their source like --T.p. or --T.p. verso or --Half t.p.. I don't know if it's specifically an RDA rule, but now these are more frequently spelled out like --Title page or --Title page verso or --Half title page.
A copyright date should go into its own 264 with second indicator 4, like: 264 _4 ‡c ©2007 (the ‡c will be its only subfield, and there's no period after it) If that's the only date in the 264, you can replace it there with a supplied date (in brackets): 264 _1 ‡a Washington, D.C. : ‡b National Academies Press, ‡c [2007]
When publication data was unknown in AACR2, we used latin abbreviations to indicate this. If you see "S.l." in 264‡a, that should be [Place of publication not identified]. Similarly, in 264 ‡b "s.n." should be [Publisher not identified]. There are other abbreviations used like [et al.], [sic], etc but what they should be replaced with may hard to recover from the record content. Another big barrier to a perfect conversion: so much that we used to record (like always "Calif." for California in place of publication) we now transcribe ("Calif.", "Cal.", "CA", "California", use whatever's on the piece) but I think it's nice to automate what you can, like you're doing!
Also you may notice in a lot of old records, you have one big set of brackets that span the whole field (or set of fields even), like: 260 __ ‡a [Place : ‡b Publisher, ‡c Year] But these fields should each have their own set of brackets: 260 __ ‡a [Place] : ‡b [Publisher], ‡c [Year] This is actually an ISBD change, not an RDA change, but it hit at about the same time. It made more sense on catalog cards when people would be looking at the whole thing at once, but makes less sense when all fields are indexed separately.
The GMD (245 ‡h) should probably be removed, as it is part of AACR2 but not RDA. (Some libraries are still keeping this for the moment until their OPACs figure out what to do with 336/337/338, but it's also going to start disappearing from OCLC records too, so its days are numbered.) Removing it is a little tricky, as you'll want to keep any ISBD punctuation in the ‡h that precedes the next subfield and keep it in ‡a, so:
245 00 ‡a Cancer in elderly people ‡h [electronic resource] : ‡b workshop proceedings
should become
245 00 ‡a Cancer in elderly people : ‡b workshop proceedings
It may help you determine 336/337/338 as well.
336 (Content Type) is a required field, but did not end up in any records I tried. You may be able to determine this from other parts of the record. For example, I'm thinking that you can conclude that you've got content type "text" (txt) if the 300 only says how many "pages" or "volumes" you have, but I'd have to check the rules more carefully to be sure. Other types tend to qualify that, like "1 score (40 pages)" or "1 atlas (80 pages)". GMD should also help, but it's weird because it's really a mix of 336/337/338, so it might give you a clue to only some of those.
RDA carrier types (338) are grouped by media type (337) and the codes that appear in their ‡b reflect this. So if you're using 337 unmediated (n), the 338 should be something from the unmediated carriers list (code should start with n) (reference here) You might also have good luck determining this from the 300 or the fixed fields.

Please let me know if you have any questions about any of these!

RDA Conversion #5

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