Im changing from WEP to WPA2 security on my home wifi network. Can I use the hex WEP key as a WPA passcode when converting from WEP to WPA, to eliminate the need to update all wifi connections with a new passcode?
No, there's no guarantee that your client devices will just auto-upgrade their security and keep using the same key even if you don't change the key.
In fact I would expect most clients to require you to re-enter the key even if you kept it the same.
By the way, when you make the change, go straight to WPA2-only (AES-CCMP only). Don't keep any form of original WPA (TKIP) enabled. There were vanishingly few devices that ever supported only WPA without quickly getting upgraded to WPA2, and any such devices that ever existed have long since been retired. Leaving original WPA enabled alongside WPA2 just complicates things for your network and has no benefit.