Yes — the Portion Model dropdown is editable on an existing plan, and switching it immediately re-filters the Group dropdown and shows/hides the Portion Size field.

Be careful, though: the portion mix itself lives on each price row, and switching the plan's Portion Model does not touch it. After switching to Customize, existing price rows show Default until you edit each one. After switching back to Preset, the Portions column (and the only way to view or clear a row's mix) disappears from the admin — but in the current version, any portion mix a row still has is not actually cleared: it is still copied onto new subscriptions built from that price, and any category left with a single portion size in that stored mix is still silently applied when the customer's meals are chosen. So a plan that now looks Preset in the admin can keep fixing some categories' portions behind the scenes for any price row that still carries old Custom data. Clear or recreate the price rows before switching modes if you want a clean Preset plan. Subscriptions already created keep the portion information they were created with. The Portion Model option itself is only offered when Meal Plan: Portion Size per Category is enabled.