Follow the below steps to change the Account.

Option 1:

1. Open the Contact record.
2. Use “Disable Partner User” to disable the Contact as Partner User.
3. Now change the Account.

Option 2:

1. Open the Account to which the Contact has to be mapped. Select “Enable as Partner”.
2. Select Yes, I want to enable this account as a Partner Account. Click Confirm button.
3. Now change the Contact’s Account.