When I imported some in 3.9 i got a message saying:
"Class (Folder)" exists, would you like to keep the current one or import the one from the package?
I don't remember seeing that in previous versions though. Beware that I have seen some significant disadvantages with non-ez created packages - specifically templates are added to the "standard" design, and many extra overrides are packaged up along with the full view template and line view template (like flat_top, pagelayout, and header) even though they are unrelated.
I'm using version 3.7.6 on this particular site, I've been somewhat reluctant to try an upgrade because the site is live (and still in development) and I have found this version to be very stable.
Is there any way I can download the weblog package raw and then manually create the required content classes and objects and upload the required files perhaps? Not that this sounds like a fun idea, but it might help evade problems!
Maybe an upgrade is the best option, but it really worries me that there will be issues during or after the process, and I think the client will have my head if the site has to go down for more than five minutes.