Yes, you can just edit the engine/settings.php of 1.9 to point to an existing database.
If location of the datadir or the site URL are not the same, you must update them as instructed here: http://docs.elgg.org/wiki/Duplicate_Installation#Database_Entries
Ok great, thanks for the quick reply. I guess I'll give it a try then.
Remember to keep a backup of everything in case something goes wrong during the upgrade.
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