This is specific to Oregon 550 units and the 25k combined parks and trails maps which are an extremely useful resource. People have reported conflicted experience with this GPS showing 25k maps. I tried for ages to get these working with a map of all of the UK at 50k and obtained a variety of inconsistent results. Now what I am about to say sounds bizarre and I do not claim to understand it, but I have confirmed that the results are 100% repeatable. This with the latest firmware for the 550.
Having read the thread I changed the priority of the 25k combined maps from 20 to 21 but was still unable to see the 25k maps - the 50k map was displayed at all scales. To make it work I have to erase my SDHC card and then copy the 25k maps first and then copy the 50k map. It is the order of writing the maps, not their name (and I suspect the draw priority but I have not confirmed that) which is critical to achieving the normal change of scale at 300m. Also the card must be erased rather than just deleting the maps and copying them in the correct order. I think this may be a "feature" of the 5xx series. In the map settings on the GPS unit the 25k parks and trails must appear above the 50k map for it to work properly.
This explains the otherwise conflicting reports. For example the report that the maps would not work with the V3 50k maps but did with the v2 version is nothing to do with the version but the order of writing the files. I have success with V2, V3 and V4 50k mapping with the combined parks and trails 25k maps. I hope that this may help anyone who is being driven to distraction by this. I spent an entire week being driven mad by this problem.