Thank you for your reply,
as for 1) - hopefully it's all right, but I'll also highly appreciate an answer from one of the developers of ISPConfig, because it's a production server I'm talking about, and I am a little bit nervous about making experiments.
as for 2) - in some cases I have a situation when, say, three sites belong to a client. I have allocated, say, 1 gb for all these sites totally. I want the client to be able to split this 1 gig of hdd space among his 3 sites. In the current setup (as explained in my first post) it's not possible because the client can't change each site's limits imposed by admin. So best of all for me is to allocate some total hdd space for CLIENT - and let him manage the rest.
That's why I'm wondering if it's possible to change ownership without recreating everything from scratch.