<?= is a shorthand for
<?php echo $a; ?>. As far as the short syntax is supported by all the servers where the application is deployed, there are no technical reasons not to use it.
So it comes to personal preferences. Previous developer seemed to prefer shorter syntax. There is nothing wrong in that. You seem to prefer the explicit one; this has nothing wrong either.
What is wrong, however, is the inconsistency you're introducing in the code base. As with any style choice, it's much more important to be perfectly consistent than to chose the absolute right one. Therefore, don't mix both.
If you decide to change the style everywhere, well, you can. Unless you work for a company which pays you to deliver value; spending your time changing the style doesn't deliver value, and could potentially introduce bugs.
So what should you do? Either keep your code consistent with the existent code base, or discuss the change with the stakeholders, so they could decide if the benefit of the verbose syntax outweighs the cost of the change (and the cost of solving bugs possibly introduced by the change).