I would like to update my limesurvey server.
Source version is 1.7.2
Target version 2
So you are going to update from a very, very, very, very outdated version to a very, very outdated version?
Both versions are not supported anymore. And there are various branches in version 2.
Anyway, I would highly suggest to update to version 3, which is currently the only version that is supported and received updates.
As Louis suggests, I would also go for a clean install. Your current version is so old, that it is probably not clear what can happen during update and I also think there is no one willing to look into fixing things for these old versions. Just doesn't make sense.
Go for 3.x and don't look back.
Of course, if you have many surveys and many users this might be some work, but I don't think it makes any sense to go to V 2 (which ever of the 2 branches).