Is work site change amendment required before joining new employer ?

My change of employer H1B approved with NY location but I couldn’t join due to personal reason. I’m still working with my previous employer. Now the new employer agreed to a different location for the same client.

Can the new employer file an amendment before joining or can they file an amendment only after my joining?

Can I join in the new location without amendment or should I join at the approved location first?

You can join the approved location without any problem .

But to change the location , if the location is outside the Statistical Metropolitan Area (SMA) than the original approved LCA by the new employer , then YES . There should be an amendment . But your new employer can file this and while they await am approval you can start at the new location .

Assuming other than the location change all other job aspects remain same . USCIS website defines when should an amendment be filed .

https://www.uscis.gov/news/alerts/uscis-draft-guidance-when-file-amended-h-1b-petition-after-simeio-solutions-decision