The correct way to do this is to make a modified copy of the Footprint, assign that to the Package attribute of the relevant symbol(s) in the schematic and then do Update PCB. You could try the "Lock" attribute but I think that only applies to physical xy location.

You need to understand that In EasyEDA the schematic is the master document from which the PCB is originally created and then subsequently updated, not the other way round. This is explained in more detail in (2) in: [https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a](https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a)