When a meeting is saved under a project there reference of meeting entry doenst change if the meeting is saved again under another project.
(This issue came up because the user selected the wrong project during the creation of meeting)

Yes, reference is defined to be unique, and may not be changed (by default).
There is an option in Global configuration screen to have the reference changed on type change.
1) this parameter should also include project change... => Ticket #1279 recorded
2) if this parameter is set to "yes", you can change type of meeting twice (to get back tpo initial value) to force recalculation of reference
There is also an administative function to force recalculation of all references. Have a look at the administrative screen.

The consequence is evident. If a user does a search of all meetings associate to the project this meeting doesn t appear because the reference is wrong;
(for information we put a number in the name field of project to make easy the search process)
How could we find all meeting which belongs to the project?

The easiest way is to use the project selector (field on top left of screen) : it will automatically restrict visibility of all items belonging to selected project.
You can also use a filter on project...

Thanks.
As usually perfect answer.

Regards,
Nicolas