I called up CASA with a similar question this week and they were surprisingly helpful, so try that. You shouldn't have to do a full on initial again, but you will have to do the age related tests, so might pay to check those and renew at a strategic time (eg if I renew before my next birthday I won't need to do any test, but will if I renew after).
The key dates are: you must get your Class 1 renewed within 28 days of expiry if you want to keep your original expiration date. You can renew after this but the date changes.
You must renew your Class 1 within 3 months of your expiry if you want the DAME to be able to revalidate your medical on the spot. If you renew after this date, it has to go through CASA for approval, they said their processing times are currently about six weeks.
Can't speak to extra rules for over 60 year olds.