if you feel you need more time and are going to be late for a deadline, request for an extension, cite the amount of extra time you need, and stick to your new deadline. some editors may be miffed, but they will always appreciate the courtesy. this will also give them the leeway to work around the change—lay out other stories first, look for a filler just in case, move your story to the next issue, etc. in some cases, they might also welcome the delay because of a new development and encourage you to take another angle.
besides, it’s good form to always inform someone if you’re going to be late and what time you can make it (just make sure you keep your word!). at least in this aspect, may breeding ka. kahit balahura ka talaga. hahaha.