Follow steps outlined by @Rea_M to batch recategorize although things change but I used to be under the impression that this capability was exclusive to the accountant and/or Advanced versions.

Rest assured, however, that nothing you change in a reconciled transaction other than date or amount will have any effect on reconciled status. You could change the "paid to" or expense category from months ago with no I'll effect.

You can set a default COA for vendors or set banking rules (which tend to change at will for no apparent reason) but more important is to Review each and every transaction in your banking import and -even though allowed by choice - do not select any automatic posting without review