You can't -- at least not directly. The Average Cost applies on a security basis to all transactions for that security over all time. If you just toggle that box on, all historical sales will switch to using average cost impacting realized gains on all prior sales in all accounts. Before going on, I will point out that Vanguard will do by-lot specification for sales, if you tell them to. I do not know what their policy is on making a change from average cost to by lot. I have a concept in mind that may be a reasonable work around, but I would like to think through it a bit more before going public with it. The gist will be to create a new security such that the old one would apply for prior historical sales and the new one would apply going forward. Give me a couple of days to cogitate on this.
I presume that the intent is ...
My second "surprise" was how Quicken handles changing my holdings to ACB. They made known that ALL "BUY" lot information would be discarded with the change, FOR NO DISCERNIBLE REASON, but inferred that they would explain how to retain that info - hmm!
Thanks, Q.L, for your thoughtful response. The whole topic of cost basis is clearly more complex than one might expect Quicken designers to be experts in. I understand "by lot" (FIFO?) better than average cost but plan to quiz my broker on Vanguard's selection rationale for using average cost as the default. Meantime, thanks also for offering to delve into a way of retaining lot information that Quicken promised to deliver in their "Help" link. Meantime I may explore having my broker change from average cost, if the IRS will even ALLOW it on an investment previously sold on that basis.
Final note - it appears that whichever cost basis I use, Quicken appears to record share sales in FIFO mode, ...