Are these fields currently mapped to any Marketo fields, or are they just synced (so they live as SFDC Custom fields) in Marketo? It's a bit simpler if it's latter, as you don't need to worry about changing the types of the corresponding fields in Marketo (you wouldn't need to do for picklist → multi-picklist as both are represented as strings in Marketo, but you'd need to change the synced Marketo field type to text area in case of text → text area change in Salesforce)
- Piclist to Multipicklist: When a piclist field is synced to Marketo, it maps to a String field, not a constrained picklist. Existing Smart Lists using “is” logic may stop behaving as expected, and you'd likely need to switch those to "contains” in the smart list/smart campaign.
- Text to Text Area: Marketo treats both as String fields. However, Text Area supports larger character limits. If Salesforce increases field length beyond Marketo’s limit (typically 255 for standard string fields), truncation would occur. Again, this applies if this field is currently mapped with a Marketo text (string) field.
If you choose to change the field type in Marketo (to match Salesforce's type, especially in case of text to text area), the field must be removed from use in Marketo (it cannot be in any smart lists, campaigns, etc), and then you will be able to change its type. Keep in mind that if you change a field type and there is data in the field that is incompatible with the new type, the existing data will be removed.