Mixing MapSource and BaseCamp

I have managed to send all of my mapping products to my 450 with Mapsource - no problems. The only maps I cannot use in Basecamp are my Bluechart Atlantic charts.

Mapsource should work fine with your Oregon (or at least it does with my Oregon 400t). When you install Basecamp, it will also be able to access all the maps you already have loaded into Mapsource.

However, I would have thought that you'd know City Navigator maps are locked to the first GPS they are installed on and cannot be moved to a different unit. So if you are using these City Navigator products on your 60cx, you won't be able to install them on your new Oregon. Garmin will not transfer your license to a different unit. Usually the best approach is to sell your maps along with your old GPS since they won't be any good with a new unit.

OTOH, any unlocked maps you may have in Mapsource, such as US Topo 100k or free maps from GPSFileDepot can be installed on the new Oregon.

Your comments about the City Nav maps being locked to only one unit are interesting. I am pretty sure I use one set of MapSource maps for both my 60CS and my iQue

That is highly unlikely, assuming that you have legitimate copies of City Navigator. From Garmin's knowledge base:
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MapSource Unlock Policies

Your MapSource coupon or registration code is valid for use with your own Garmin GPS unit. This agreement provides a one-user license, and it is illegal to share this data with others.

If you would like to obtain additional regions or utilize more than one Garmin GPS unit you own, you will need to purchase another code for each unit. (The exception to this is with MapSource BlueChart CD data. The regions you have unlocked to one compatible Garmin unit you own from a specific BlueChart CD you may unlock to ONE other compatible Garmin unit you own for no additional fee.)

Important note: Once you complete the unlock process, Garmin cannot make changes to your coverage area, transfer unlock codes to different Garmin devices or give refunds or exchanges for MapSource unlock codes. Unlock codes are GPS unit specific and cannot be transferred to another GPS unit. Please be very careful to select the regions you want to unlock. Please pay close attention to the Unlock Policy statements and FAQ’s on this website for very important information on how unlock will apply to your specific situation.

The only problem you will run into is that the way waypoints are stored on the new units (450, 550, Dakota, 78) is different and Mapsource is not able to find them through the "normal" method.