I have a 2007 pre-facelift Octavia Combi, which has quite a large dent in the boot lid. There is not many pre-facelift trunks for sale, so I have been thinkin if a trunk from a facelift model would fit in the 2007 model car?

According to the parts catalogue the Octavia mk2 pre- and post-facelift rear door / luggage compartment cover (called a 'lid' in the catalogue) is the same.  Note that this is the bare door; I haven't cross-checked the fittings but they're available via the same links.
Octavia mk2 pre-facelift Czech manufacture 2007 combi/estate rear door HERE

Octavia mk2 post-facelift Czech manufacture 2009 combi/estate rear door HERE

 

You may already know this but older Octy2 'lids' often have bubbling rust issues across the bottom panel or around the number plate lights.

 

If you do manage to find and swap doors please let us know.