by Colin Fraser.
Hi Deepti, yes, there is an issue upgrading user data from v1.9.x to 2.4 and that is probably because the database changed significantly from v1.9 to v2.0, then an upgrade package was released for v2.1.x which worked to v2.2.x and now does not work anymore.
There is a number of Moodle Tracker items already, but the one way of making this transition is to do an upgrade to v2.2.x then to v2.4. I know it is an inconvenience, but it also means your user data can be successfully included in your later version. I suspect the this is similar to previous Moodle updates.
The idea now, of course, is to remain relatively current. say upgrade once per year instead of every few years.