check this issues
and you can try with this plugin,
In Jira Cloud, (as at February 2021):
View the current parent ticket. Above the table of child issues, there is a '...' button. Click the button to see the menu; and click 'Bulk edit'.
You should be taken to a multi-step wizard process in which you can select the child issues you want to move, then decide which new parent ticket you want to move tickets to (and potentially change issue type too), and action the change. (For some reason, when i just did this it skipped the file-selection step but you can go back to that step).
I realize this has already been answered, but if you want to do this with native JIRA rather than a plugin, you can do it in two steps:
For Jira Cloud:
1. Open the parent item to display the linked subtasks and decide which parent issue key you want to move the subtasks to.
2. Copy the issuekey numerical portion (trn-113) as 113 for all subtasks to be moved.
3. Figure out what the project 'prefix' is (the acronym for the project name that appears in front of all issue id's for your project: If 'TRAIN' is project then 'trn' may be the prefix. (you chose this when you created the project).
3. Click the search tool (magnifying glass in upper left of 'management column area?'.
4. Don't enter anything on the search entry field.
5. Click the 'Advanced Search' magnifying glass.
6. type (without quotes) into search field: issuekey in ()
7. past in all the issue key numerical portions you copied: 113, 114, 156 ,199, 333356, etc. between the parens: "issuekey in (113, 114, 156 ,199, 333356)".
8. repetitively insert the project 'prefix' and comma+space delimiter : ', trn-' between each numerical key portion; issuekey in (trn-113, trn-114, etc...)
9. select 'bulk change' in upper right of search screen.
10 proceed as usual with 'bulk change'.
11. choose all issues in 'next' screen, click 'next'.
12. choose 'move' in 'change operation screen', click 'next'.
13. (a) choose project to move to (not), (b) issue type to change to (NOT), (c) parent issue to attach to.
14. check checkbox titled: 'Use the above project and issue type pair for all other combinations.'
15. Click next: all subtasks will be attached to the issue you selected in #13. step (c) above.
16. Wonder what else you may accomplish using this method.
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