There are several factors at play. First, of course, is whether or not the current end caps are those which receive a 5mm hex wrench. If so, then you can move on to the next step. If not, then either a special press,or Phil Wood, will be needed.
Next step, make sure the drive-side end cap (if you're going to replace it) will provide enough depth for your freewheel. Different freewheels (5,6,7-cog, compact or "regular") require different depths of drive-side end caps. Once you've measured the depth of the drive-side end cap, you can determine the necessary non-drive-side end cap by adding the drive-side end-cap's depth to the width of the hub shell (I think it's 72.5, but you can measure it). An example:
IRD 7-sp. FW uses 28.5 drive side, and 35mm non-drive side.
I have lots of end caps, and would be happy to supply them.
Rich