It is possible to edit Eagle Files directly since they are xml files.
=== Change font on layers 25, 26, 37 27 and 28 to vector ===
Regex replace:
<code>(<text.*)layer="(2[5-8])"></code>
with:
<code>${1}layer="${2}" font="vector"></code>
=== Change font size on layers 25, 26, 27 and 28 (0.6096 --> 0.8128) ===