We may be involved in switching an SFDC instance (downgrade to Enterprise). Past posts seem to indicate that this would require Marketo to issue a new instance and that there is likely to be loss of activity data (that isn't written to SFDC). Is this still the case?
Yes, if you change your Salesforce, you must switch to new Marketo. Our team at Salytics is doing this for a client right now. There is a way to save your activity data though time and money will be factors on the decision to do this.