Some thoughts about the quality of your existing denture

If the dentures are old and do not really fit properly, the dentist might advise to have the lower denture either remade, relined or rebased prior before the implants is being replaced.  A rebase is when the complete replacement of the pink acrylic  base is being done by a new one. 

This makes really good sense because a firmly retained lower denture, not fitting the right way with the upper denture will dislocate it and make the upper denture unwearable.  Also if the lower denture has been repeatedly been repaired, or the teeth keeps falling out, then any modifications necessary to allow the same lower denture will weaken it further and make it even more vulnerable to break in the future.   If your dentures are older than seven years, the patient should think about having a new set made prior to placing the implant, or shortly after.

Many surgeons start to use minis as abutments for crowns for lower incisors and upper lateral incisors.

Never fix any implant to natural teeth, including minis.  Osseointegration attaches the bone to the implant, the periodontal ligament attaches to natural bones.  The periodontal ligament allows having the implant to stay healthy.  If you split natural teeth to implants - this can cause the natural tooth to ankylose.   An implant splinted to a natural tooth may not hold up as a result of the movement of the fixed splint.

Partial dentures may also be aligned to both implants and to natural teeth without any risk factor.  

The dentist might suggest investing in a set of models, showing what happens to a mandible after teeth extraction.