I suggest the following script for transferring all Jira data from one username to another username (merge the user names):

import com.atlassian.jira.component.ComponentAccessor
import com.onresolve.scriptrunner.parameters.annotation.*
import com.onresolve.scriptrunner.db.DatabaseUtil




@ShortTextInput(label = "Extra User name", description = "")
String oldUserName


@ShortTextInput(label = "Target User name", description = "")
String newUserName




def getUserKeyByUsername(String username) {
    def userManager = ComponentAccessor.getUserManager()
    def user = userManager.getUserByName(username.toLowerCase())
    if (user) {
        return user.getKey()  // This returns the user_key for the given username
    } else {
        return null
    }
}






if (oldUserName?.trim() && newUserName?.trim()) {


String oldUserKey = getUserKeyByUsername(oldUserName)
String newUserKey = getUserKeyByUsername(newUserName)




if (oldUserKey==null) { return "ERROR: User key for $oldUserName was not found. " } 
if (newUserKey==null) { return "ERROR: User key for $newUserName was not found. " } 


def SQLList=[
"update jiraissue set assignee=? where assignee=?",
"update jiraissue set reporter=? where reporter=?",
"update jiraaction set updateauthor=? where updateauthor=?",
"update jiraaction set author=? where author=?",
"update  changegroup set author=?  where author=?",
"update searchrequest set authorname=? where authorname=?",
"update searchrequest set username=? where username=?",
"update sharepermissions set PARAM1=?  where PARAM1=? and sharetype='user'",
"update project set lead=? where lead=?",
"update component set lead=? where lead=?",
]






 SQLList.each { query ->
        DatabaseUtil.withSql('jiradb') { sql ->
            sql.executeUpdate(query, [newUserKey, oldUserKey])
            log.warn("Executing SQL: $query with newKey=$newUserKey and oldKey=$oldUserKey")
        }
    }












return ( "$oldUserName  merged into $newUserName ") 

⚠️ WARNING: This operation is sensitive to security and data privacy. Ensure that both usernames belong to the same individual. Merging data for different users could result in security breaches or privacy violations.

Automation for Jira, which uses Jira users as actors and rule owners, and the software house behind which has already been bought by Atlassian, has a feature that seems to address this problem: Transfer User. I haven't tried it yet (the doc is only for the cloud, so I'm not sure if the button I can see in the Server version does anything), but if the coders who make Automation sometimes work on Jira proper, we might actually see this feature added in less than another 17 years

576ed7c5e148 YouTrack indeed supports user merging – when you delete an account, you're asked to choose a replacement account. (And only if you choose the guest account as a replacement, the references to the deleted account are replaced with Deleted User). I've used this feature many times, never had any problem with it.

A good catchy slogan yes - another one would be "Atlassian - ...."

Atlassian's new company slogan should be:
"Atlassian – We don't support that feature and have no plans to work on it in the foreseeable future"

FWIW

Linking these two tickets

https://jira.atlassian.com/browse/JRASERVER-3132

and 
https://jira.atlassian.com/browse/ID-240

There's YouTrack, which does support user account merging (based on a quick Google check). Its fairly similar to Jira from my limited experience with it, just in the Jetbrains "ecosystem" rather than the Atlassian one.

Any suggestions for Jira alternatives?

i have been using RT from BestPractical which i in 2003 found to be very very powerfull, and i am looking in that direction again. However, migrating a 16 year old jira legacy is gonna be a long run..

Any suggestions for Jira alternatives?

No review since 30 March 2018?

im sorry laurent, but this issue has been parked in atlassians pseudo-closed issue void called "not being considered". They claim they'll wait a year between reviews of issues in this status, however in reality here on planet earth that year is more like a decade, and the review is just another laugh from the moneymachine called atlassian.

 
This issue was raised in 2004 - back then i had been using other issue trackers that have had this exact feature since the 1990ties.. however some 20 years later, Jira still has not got it. 

..... abandon ship if you can, migrate your Jira into something serious