Make sure your new EHR system can easily import the data from your previous EHR. This should be an easy process that simply requires you to provide your new EHR with an .xls or .csv file.
Ideally, the new EHR will allow for the import of other pertinent patient information like medication and allergy lists along with lab results. This data should be in the industry standard C-CDA format.
Finally, your EHR should allow you to export your patient data in the case that you decide to discontinue service and move on to another system.