Well, filming is done for the next film, so if her fate was already planned, they'll be okay. But if they had to do more, I'd rather they recast than the digital makeup they did for Tarkin. I thought it looked good, and made it seamless given how close the first movie and Rogue One were, timeline wise, and a great way to honor an amazing actor (the guy who did the physical acting played Sherlock Holmes once...and did so by studying Cushing's performance of the character), but part of my brain is saying "too soon".
Also, to nerd out for a minute, becoming a force ghost is a technique that must be learned through jedi training. No indication that Leia had any, or was even force sensitive, other than her family being jedi.