Depends primarily on how Item Groups and Item Types are used in your environment. I think the main concerns will be in analyzing, reporting and planning.
For starters, do a search on the Baanboard for "change item type" and "change item group". There are several threads that will give valuable insights.
When changing Item Group, the biggest issue is whether the Item Group is used in mapping the integrations to finance. If it is, you must analyze and perhaps change the mappings. Otherwise, reconciling postings to ledger accounts and dimensions will be a nightmare at best.
Changing Item Type has implications for cost calculations and cost updates. Be sure that you an appropriate cost structure defined for each changed item and that you understand the cost impact of changes for any open orders (PUR, SFC, SLS, etc.).
My focus (and bias) is primarily finance so other members will be able to offer better advice outside of costing but I think you'll also want to review and change the planning types.