FYI do NOT
delete a previous version of a map product before
doing an update. The updater will only download and install it again anyway (doubling the already long download/install time). I've no idea why since, as already stated, updates are not incremental.
In these days of hundred GB drives it shouldn't be an issue having two versions of a map product on your HD temporarily.
Once you have updated maps you can then delete the previous version to free up disk space.
BTW the map updater downloads everything to the TEMP/TMP folder. For most users this is likely to be on C: along with their maps. That means you need 3x (2x as above + temp) of a map product space to perform an update. Also the data in the temp folder isn't deleted after the update--at least not the last few times I've done one. You'll therefore have to do some cleanup yourself if disk space is limited.