Comment 12 for bug 401205

You cannot change the type of an account.

For what you want to do, you can either create an asset account, which you include in your pay transaction, and later you transfer money from that account to your 410k account (which should be an investment account), or you can create a brokerage subaccount in your 410k account, and you transfer your money from your pay transaction to that brokerage.

Either way, you should take a look at the KMyMoney manual, the Accounts and Investment chapters will be of use to you.
http://kmymoney2.sourceforge.net/online-manual/index.html