You can not change the conditions of the issued Extension of Stay during the 1 year validity.
If your current extension of stay is ending and you need to apply for a new 1 year you can use the monthly income method but you need to proof 12 months of monthly foreign deposits prior application date, if that Extension of Stay is issued you can use the 800.000 Thb from your back account.
You can NOT transfer an Extension of Stay which you got with help and documentation from a specific employer to another employer in another district in Thailand.
You go in advance to the Immigration Office where you got this Extension of Stay and cancel this on your last day of working or the date you agreed on with your current employer. You also cancel your work permit on the same day.
Than you travel to Chiang Mai and apply with your new employer for a new Work Permit and for a NEW Extension of Stay at the local Immigration Office. It needs cooperation of both employers and both Immigration Offices but it can be done.
IF they deny your application, they hand it back and you can stay until the permitted up to date stamped in your passport.
IF they NOT approve your application you get a NEW stamp with the text Application not approved and 7 days to leave the country, this is normally 7 days after your applied and not an extra 7 days on top of your current stay.
It is up to the decision of the Immigration Officer but the option is still there until 25th July (date edited, thanks Lisa)
Land borders to neighbor countries are open, airports are open, some reports are that your application is denied and just handed back or they take your 1.900 Thb and stamp your passport application not approved and give you 7 days to leave the country.