First of... big a$$ HUG.... breathe.... she is your baby and you ave right to worry whenever surgery is spoken of.
Banks has HD, very bad infact, she was diagnosed with it at around 8 months and at the time, our normal vet was on vacation and the vet that told us actually said the words, 'you will need to put her down at some point around 2 or 3 yrs old casue she will not be able to walk' -- well, Banks is now a very active 8 yr old - yes, she is in constant pain and is on daily meds to help her have a good quality of life. As you stated, when it is cold, she has issue and slowness in gettting up or moving around after sleeping, but she is fine after a few minutes. Our regular vet, Joe, has never, not once, mentioned having to do surgery for Banks, he told us as long as she is walking, running and able to activitly move around on her own there is no reason for it. If Joe were to say she needed it, we would do it in a heart beat, but he prefers to treat her via pain management.
There are other cases where the surgery is totally necessary regardless of age, I think you need to get a second opinion and really determine if now is the right time or is it necessary at all. I think @sheshistory or @lexterwayne may have had hip replacement for one of thier babies and might be able to give more insight.