I know that if you exceed $50k in your Investor shares holding, eventually a button will appear on the VG site that allows you to convert them to Adm. (And eventually VG will do it themselves.) But I didn't want to risk losing the window I had with VHCOX in my IRA (bought $25k just before 12/31 and after 1/1, maxing out the annual limits), so I called them to do it manually while the balance was over $50k.
VG is good at this—they actually did it while I was on the phone!
I have a much older VHCAX position in my taxable acct (and took a nasty CG hit last year from it). I will also add $25k there—but after a couple of hot days, I will wait for a dip.
POAGX may be a better fund, but I did not get in before it closed. I doubt any of the closed Primecap funds will ever reopen.