re xmp files & metadata updating

[Fiona & Paul, Jakob, Lucas and Finlay Jefferies (pandfi)]

I'm looking at my meta-data and I have data in some older files but have 
moved to xmp files now.

1) I want to make sure all the meta-data is in the xmp files. Is there a 
way to do this - using the sync tool doesn't appear to work (it refers 
to copying data from-to the database specifically too so I didn't really 
expect it to). I've got a hack whereby I add a temporary Tag to all 
pictures, apply, then remove teh tag, this makes sure that a new xmp 
file is made if none already exists. But I have 25k photos and writing 
the xmp files is very slow; so is there a better way?

2) Some of my xmp files are xml, some are xpacket (which appears to be 
the newer format in digikam terms). I'd like to harmonise these to one 
format, is there a built in way to do that?

Thanks.

FWIW this is a prelude to updating to Digikam 5 (on Kubuntu), which I'm 
hoping will have better face recognition. Currently digikam seems to 
just stick all the faces recognised in to the folder for the most recent 
manually entered name; which is proving somewhat annoying!