Will Botox work on lines that are already formed?

In some cases, yes. This is actually a great question that comes up every day in my practice. Those imprinted lines develop over many years of repetitive muscle contraction. Botox works by reducing muscle contraction. Consistent use of Botox over 6-12 months will often lead to improvement of those lines that are folded into the skin. In some cases, a small amount of filler will also be required to lift and plump those imprinted lines. However, in most cases, it is worth trying Botox first prior to considering fillers. Seek an experienced physician injector for further discussion.

When lines (creases) are present at rest, there already has been some thinning of the skin in those areas. Botox (and Dysport and Xeomin) will relax the muscle deep to the lines but will not restore skin cells or reverse lines. However the overall appearance of the skin typically looks smoother. 

Botox only works on wrinkles that appear when making a facial expression (dynamic wrinkles). I suggest getting an assessment from an experienced and reputable medical injector to see whether fillers or Botox would be most suitable. However, it sounds like fillers would be better to address your concerns.

BOTOX® Cosmetic is effective both for preventing the deepening of wrinkles as well as the treatment of established wrinkles. It works by controlling the wrinkling of skin produced by repetitive muscle contractions that, over time, will result in lines even at rest. The degree and rapidity of improvement depends on the depth of the established lines. When the lines are relatively shallow, improvement can be evident within a matter of days. For deeper lines, however, it may take months to achieve the maximum amount of smoothing. The key is to maintain the relaxation of the muscle with a consistent treatment regimen so that the lines don't begin to reestablish themselves.

Botox works beautifully well on wrinkles created by the movement of our facial muscles during facial expressions.  The skin above these muscles wrinkle during movement, and with continuous wrinkling they eventually become "etched" into the skin.  Even when these lines have created creases that are present at rest, relaxing the underlying muscle will help these lines to soften and prevent them from deepening.  Occasionally superficial injections of a filler will be needed to achieve full correction.