No, it's too early for you to be worrying at all.
Even though the information would be available to them from your DA claim, your IP claim must still go through all the necessary checks for control purposes, & the application is treated as a totally separate application, as it is a totally different scheme administered by a totally different section.
The chances are that based on the information that your GP sends to the medical assessor that this will be enough for him/her to make a decision. Then again it may not be.
Considering that the medical criteria with regard to what type of employment that the medical assessor considers you are/are not, will/will not be capable of performing in the future, is different for IP & DA, an assessment has to be made by them. This assessment could be either be decided without you being called for a desk assessment or based on existing information together with the latest report from your GP, so don't worry or stress out over this as yet.
Also, your existing DA claim may have beeen flagged with a review date of (for example) two years. You have been in receipt of it for 18 months now, so the existing records that they have for you may not in reality cover you as being unfit for the next 12 months (which is the requirement for IP).
Even after your IP claim has been approved & it is in payment, it is likely that there may periodic reviews of your claim to verify ongoing entitlement, & you may be sent a form for your GP to complete. This too is nothing for you to worry about & is purely for control purposes.
If you are receiving a Fuel Allowance at the moment you will more than likely be means tested again for it to be paid in conjunction with your IP payment, again because it will be associated with a different scheme, but you will still receive it with your DA payment until such a time that your IP claim goes into payment.
The above may not be the exact situation in your case, but my point is that there could be very simple reasons, so I'd advise you not to worry at all, let them wade through the red tape & wait for any further requests, updates or instructions from them.