Introduce a new, single-value field in the bundle, then use Views Bulk Operations (VBO) to copy the data from the old multi-value field to the new single-value.
VBO have an option to "Execute Arbitrary PHP Script" where you can insert a PHP-snippet to do the copying, something like:
$object->field_foo['und']['value'] = $object->field_bar['und']['value'];
The PHP above is only an example. Use
dsm($object); (requires Devel installed) to figure out the datastructure of the entity object you need to manipulate.
When done, delete the multi-value field.
Alternate solution: Node convert
It is also possible to do bulk conversion with Node convert.
To use this method, you first clone the content type, then change the multi-value field into a single-value field in the clone. Node Convert let you map content field by field, so you copy most fields directly, and then copy the first value of the multi-value field into its corresponding field.
The disadvantage with this method is that the machine name of the type changes, so you may need to fix that in a number of places if it is used a lot. The advantage that you can do it all in the GUI, no need to write PHP.