There is less room on them now they are operating as a SFN. I would have thought that the only time they got close to full capacity would have been when they were being used for BBC Red Button streams. As far as closure goes, Arqiva will probably want to hang onto them as long as possible, they don't seem over keen on converting their main multiplexes to DVB-T2.
No commercial operator will want to switch to DVB-T2 until the main channels want to as they don't want to lose viewers. So once BBC/ITV are ready. All the remaining DVB-T will probably switchover "at once" (i.e. in stages over a matter of days) like Digital Switchover.
All my opinion though.