Tune each string a little at a time... When you tune one string all the way up to pitch it has more tension on it than it needs... Start with the largest E string and turn it a few times then move to the next and so on... until you have them tight, but still way flat... Now you can go by half steps... tune each string up a half step and work your way up... This allows the strings to share the tension of the neck rather than one string comping ALL the tension... that's most likely why you're getting string breaking... Or it could be because you've had the strings on for so long that they just couldn't take the drastic change in tension... Which might also be the case... With metal... as I've said before it's constantly stretching when it's under tension... When you put a new set of strings on they go out of tune really easy because they've never been under tension and the molecules of the metal are realigning because they're under tension... Once the metal stretches to close to the apex of it's elasticity it will stretch at much much much slower rate... sometimes your strings will stay in tune for months at a time... however... this state is when the strings are most fragile... They're at the apex of their elasticity curve... which means a drastic change in tension will push them past their envelope and cause them to snap... This is what most likely happend due to the fact that you had them tuned to C for so long... But I would recommond getting a setup if you're planning on tuning to standard... when you normally play tuned to C... your neck is going to be bent like bow and arrow with those .13's... especially sinc you have a hardtail... Normally it's not so bad on trem bridges because you have the springs of the trem to balance out the tension... With hardtail bridges all the tension is divided between the strings and the truss rod... Causing more string breakage and a lot more truss rod sensitivity to string changes and drastic tuning changes... Take it in and get a setup... and make sure you tune one string a little at a time. Never tune one string to pitch... Go slow. This will help the strings adjust to the tension as well and will keep your strings in tune longer...

I did a project on guitar string tension and tunings for a metallurgy class last year....... haha