When those Heinz shares turn into Berkshire Hathaway shares, is the cost basis for the Berkshire shares is the amount you initially paid for the Heinz shares or the cost of the Berkshire shares at the day of the conversion?
If I sell today, will the tax I pay on the capital gains be the same as if I waited until they turn into Berskshire shares and then sell?
First of all, the deal (if it goes through) is all-cash, so the shares you own won't "turn into Berkshire Hathaway shares," they'll turn into a cash payout. The cost basis is the amount you initially paid for the Heinz shares. If you then want to use the freed-up cash to buy some B-H, that would be your personal judgement call.