If I update my software on my iPhone, will it go back to being locked?

About a year ago now, I got my boyfriends old phone - which at the time was locked to o2. I got the phone unlocked as my sim card is Orange. The man said that I had to keep it on the oldest software update, and if I updated it, the iPhone would go back to being locked to o2. HOWEVER, I accidentally deleted some of my apps, which now to get back requires the new software. Does anyone know IF I update it, will it ACTUALLY go back to being locked to o2?

Yes, it will go back to being locked to o2

Since your iPhone was unlocked using a jailbreak, it will be locked again once you update iOS. The only way to avoid this in the future will be to get a proper factory unlock from one of the big online vendors like http://www.cellfservices.com/ as local phone shops usually use those obscure jailbreaks you have on your phone now.