Change field from Multi-Value to Single-Value
Use VBO

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-&gt;field_foo['und'][0]['value'] = $object-&gt;field_bar['und'][0][1]['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.