Right - here goes!
This method will provide a file which is an extension of the old contract, and is what I assume is your intention.
Firstly you'll need to delete the small existing YM-200503-ECBOT.mnd file that you've probably created, by making sure Sierra isn't running and going into the data file of Sierra Charts.
Then right click on your existing YM file which will probably be YM-200412-ECBOT, Copy and then edit/paste this to create a copy of this file. Rename this file YM-200503-ECBOT.mnd, remembering to include the file extension!
Then next time you open Sierra you can then go to file/open new intraday file and your YM-200503-ECBOT file will be there ready and waiting. 😉
A final action will be to ensure that the new file is loaded as an historical file update in the list which is towards the bottom of the edit option on the main Sierra Chart menu. 😉
Hope this helps, and as you can probably tell, I know how you feel. This information was freely supplied by our Sierra star JonnyT, when I found myself in the same predicament. 😆
Cheers
Q