The new API uses short codes similar to GC-codes instead of GUID. The old API was a bit inconsistent.
The only Sandbox callback method I can see this far that is affected is GetBookmarklist(). Today the first argument is a GUID, in the future it will be a "BMCODE". The look exactly like GC-Codes, but starting with BM instead of GC.
For the current scripts and configs there are two solutions. Regardless of which solution I choose, I will first search all scripts and configs for strings in the format ........-....-....-....-............ where dot is a wildcard (regexp). Then I will send them to HQ to get a translation list, I can not do that myself with the current APIs.
Then I will either add a translation table in the callback method, for these only, or I will try to regexp replace inside all scripts and configs. I believe the second method should be safe enough, and also more consistent. Feel free to drop comments if you have any opinions.
I am about to run this update now. Please note that the GetBookmarkList() method needs IDs starting with BM in the future, and not long GUIDs.
As a side-note, I noticed (with help from HQ) that it wasn't as simple as searching for that regexp. It matched some images, and a lot of souvenirs, and probably something else as well. But searching for a quote or double quote followed by the GUID seems better. There is a risk that I will miss updating something. Hopefully someone will report it as not working soon then.
That's a good question. Since I am an API developer and had plenty to translate. Also other kinds of data (not bookmark lists) I emailed HQ who helped me.
I thought one could just click on the owner and browse to his lists and look there, but that web interface uses GUID as well (https://www.geocaching.com/profile/?guid=a0bcb031-d6ab-4c49-9481-f04baf7a74d5). In contradiction to when to when I look at my own (https://www.geocaching.com/account/lists) which uses BM-codes.
I will try to use the API to list all lists by that owner, that should return them with BM-codes, let me get back to you.