I am 38 yrs old and my initial Class 1 medical expired a couple of months ago, so that I am continuing my PPL flying under class 2 privileges
In which case you were probably 37 when initial class 1 was issued, so its class 2 validity wil expire either 5 years from the date of medical, or on your 42nd birthday, whichever occurs earlier.
Your class 1 can be renewed at any time, by any AME, with the proviso that once it has been LAPSED for more than about four years we are supposed to check with the
Belgrano before proceeding. Sometimes they will say that another initial must be done there.
The ECG and audio will fall due at the same times as they would have been due anyway - it is NEVER necessary to make a special trip just for them - only carried out at routine renewals.
This information only applies, of course, to the UK. Other AMC's and authorities may operate to different rules and procedures.