Using powershell to move a sub-site tree to a new site collection

I have a case where I may need to move a subsite tree from one site collection to another existing site collection.

The information I have normally seen on the web talks about creating a new site collection when moving a subsite tree.

If I am going to an existing site, then can I still do the export and import or do I need to do something else.

The manage content option is not available to me - the publishing infrastructure is not turned on the source site collection and the ramifications of turning it on are too great.

I can use powershell to perform this move.

Another situation is that the destination site collection has the SP 2010 visual upgrade, but this subsite tree does not yet have that turned on. I presume that it would be best to turn it on before I move - is there a way via powershell to turn on the visual upgrade for a subsite tree only? I don't want to turn it on the sibling trees at this time.