Changing Steam guard device without the 1 week hold.

Root both phones, use smth like titanium backup to clone the app to the new phone. If you are on android.

if you change only the device you get only 2 days. you need the same number

I've done it before (on iOS), you need to select that you've changed device and you can verify with your phone number. It might be 2 days as M0J wrote, but at least lower than 7

https://support.steampowered.com/kb_article.php?ref=8625-wrah-9030#transfer

After transferring the authenticator, any new trades or market listings will be held for a couple of days to protect your items in case your authenticator is ever transferred by a malicious person. See our Trade Holds FAQ for more information about trade holds.

Looks like there's still gonna be a hold for a couple days, still better than a full 15 days if you disable on old phone then re-enable on new phone.

The only way to do it without any hold is have a rooted phone where you can backup and transfer the secret token..

My phone was stolen recently and got a replacement SIM card from my provider with the same number. There was an option for device change, which allowed me to enter a code received by SMS and could use the authenticator right after that. Got only 48 hours trade hold.