I've done some work on migrating data from the master OSM database over to our OAM database. The tricky thing here is that when you download something from OSM, it expects any modifications etc. to be uploaded to OSM, not another database. To work around this problem I've written an tool called OsmXmlTool in Haskell. Put simply it does this. Parse an OSM xml file. Create an internal representation of the data in it using haskells Algebraic Datatypes. Then it modifies this representation following several rules; all entities has to get new unique id's etc. Afther that, the datatype is converted back to XML. This data can be loaded into JOSM, fine tuned and uploaded.
To give a idea of how much work this tool saves us. Today i downloaded a bounding box of Norway with all aviation related features. I put it trough my tool, and after that into JOSM. Some minor tweaks to some tags and uploaded. Took me around 10 minutes. In those 10 minutes i had populated the database of 3000 new entities, runways, hangars, helipads, taxiways you name it. Haskell is awesome, OSM too!
To take look at the result, go to OpenAviationMap.
If you wonder how anything is done or have any questions, pop a comment! I'm maybe thinking of making the tool generic, so that it can be used in any project that want to migrate OSM data into their own database, tell me if you're interested.