How to render an entry type of a Matrix in HTML?
If you change the entry type then you're going to lose content for sure.

If changing that field type to an entry is really what you want, great. You could do something like

{# grab the first entry that we've selected #}
{% set learnMoreLink = box.learnMoreLink.first() %}
{# spit out the url for the first entry #}
&lt;a href="{{ learnMoreLink.url }}"&gt;{{ box.learnMoreText }}&lt;/a&gt;