Do you actually need to convert from 0.5 to 0.6? Other than for editing not much has changed between the apis if I remember correctly. So possibly read only tools can equally deal with 0.5 and 0.6
I try to add contour lines to a papermap (build with mapgen.pl). I’d also try to add this lines to a garmin map later. To merge ways and lines I tryed osmosis but osmosis v0.37 doesn’t work with 0.5-Files.