Sub-chapter S corp. Paying insurance premiums on owner and booking it
as a receivable from the beneficiaries. Can I expense this? I have 5
years of premiums booked. Should I expense last years and charge the
prior years to retained earnings (was a C corp prior to S with carry
over retained earnings) or should I expense all in this year?

Not sure why you would treat an insurance expense as a receivable.

I believe you can legitimately expense the balance in your receivable
account.  Technically, there is no amount due from the beneficiaries.

All past year errors should be flushed through the current year's P&L
as expense.  It is inappropriate to charge prior years to retained
earnings.  If you do, the owners will NOT get the benefit of a tax
deduction for those past years.

It is probably best to charge the prior year to a non-operating line
item on the P&L as not to distort/misstate the current year
performance.