So I have a standard 1920x1080 15 sec ad I need to make into a 1080x1350 size for a instagram post. As I don't want to start from scratch and rebuild this edit, can anyone give me thoughts for the best workflow to do make this change? I did do a mixdown and use frameflex to make an adjust but is there a way to say change the sequence so I can make some changes to fit some of the media in the frame? I hope I have explained this in a understanding way.  Or is there a way to set the grid to the instagram size so I can just rework the edit in the original sequence?

Easiest way:

Create a new project 1080x1350.

Bring a copy of your sequence into the new project

For each clip on the timeline, adjust the Frameflex aspect ratio to 0.8:1 (4:5), Pillarbox/Letterbox (Not stretch)

Reframe as needed

There are other tricks you can use, for example saving the 1080x1350 as a preset (when you create the project) so you can simply switch to it within your existing projects using mask margins.

Hey thanks. I got it and this short and simple guide helped. 

AVID - How to change the aspect ratio on the entire edited timeline - Moody College of Communication - UT Austin Wikis (utexas.edu)