Unless the site is constantly updated (php, asp ...), I
don't think that WinHTTrack goes through & scans the site,
removes all the local files you have downloaded & then goes
ahead & redownloads the whole site,
it just scans the whole site to find out what has been
updated or added.
Anyway, use "update existing download", then  tick "Set
Options / Build / do not purge old files"
You may have to tick "Set Options / Spider / Update hack"
it may be what you are looking for, but httrack will scan
the whole site to find out what has been updated or added.