how do you easily flush out a theme to one another ?

[ghost]

hopefully, this makes sense but I'm using Shopify plus. Let's say I want to push (instead of downloading) my dev site something like shopifyplus.myshopify.com to my normal site myrealsite.myshopify.com currently when I go to packer start it doesn't flush out the old theme ID's even if I changed the ENV.json file.... basically packer just tells me I'm still on the shopifyplus.myshopify.com theme id, not the myrealsite.myshopify newist ID like desired ..

do I have to keep downloading the latest changes and post up to the real version? is there an easier way to do this ?
ps i did try the packer start --dev and still shows me old themeid