G'day, labsektor, and welcome to
Kunena.
Your problem is, fundamentally, a Joomla/SQL issue. Depending on the method that people use to migrate from J! 1.5 to J! 2.5 user ids in the Kunena SQL tables may not necessarily be changed as well.
It's important to understand that the base user id in J! 1.5 is 62 - that is, all user ids start with the number 62 and automatically increment by 1 each time a new user is added. The base user id in J! 1.6/J! 1.7/J! 2.5 is 42.
So, while your users may be renumbered as part of migrating from J! 1.5 to J! 2.5, it's also important to ensure that any references to those older user ids are, likewise, changed. This is why we recommend using the
jUpgrade migration procedure that was designed to migrate your entire J! 1.5 platform (where you were running
Kunena) across to the new J! 2.5 platform. If you do things differently then you may get different results.
Many of these issues relating to moving
Kunena from one environment into another environment have been discussed repeatedly on this forum. While it is quite achievable to lift and carry the
Kunena tables from one site and put them on another, you must also remember to copy the "user environment" as well or you need to transform the
Kunena tables so that they map to a different user environment elsewhere. It is beyond the scope of a community-driven self-help forum, such as this one, to prescribe a one-size-fits-all solution but I would refer you to the discussion
[Merged topic] How to move my Kunena forum from one site to another
You asked, "How can I update users IDs in
Kunena?" The answer is to use
phpMyAdmin, export your tables, change the user ids with some other editing tool (perhaps something like Microsoft Excel) and then re-import the changed tables back into your new site database.