It is possible to replace a boot with the axle still in place.

I needed to do this on my '71 Audi 100, and afterwards put over 20000 miles on the boot.

This was before they started selling kits with a prepared boot and special glue [at least for common sizes]

I started with a replacement boot, then slit the boot so it would fit around the axle. [used a new XActo blade that I had cleaned to remove any surface oil]

Then I removed the old boot and its clamps.

I re-glued the edges of the boot around the axle [with crazy glue] clamping the narrow [axle] end with channel locks to keep the glued joint in compression for a few minutes.

Then I slid the boot up the axle, and secured it with new clamps. I checked it periodically for the next year and it didn't fail. [the we replaced the Audi with a Fiat 131 wagon]