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Two things:
One, the dealer will probably charge you for the reprogram of the BCM. They don't have to, but they usually will charge, and in addition, the dealer may not want to change the program (see highlighted area in the bulletin).
Two, if you have access to a GOOD GPS unit, drive at a few different speeds and verify that your speedo is correct now, and again verfiy how far you are off after the new tires are installed. Mine is off by 2 mph at 70 mph (showing 70, but GPS is showing 68).
The GPS is accurate, in fact, GM tells techs if a customer complains about the accuracy of a speedo, to verify via a good GPS unit. (I say good, and I define that as one that receives at least 10 channels...12 perferred...and can obtain a WAAS satellite to for a differential adjustment.)
If you still want a recalibration, here is the bulletin:

Subject: Guideline to Change Tire Size - keywords calibration program rim wheel #PIT3271B - (04/12/2006)



Models: 1995-2006 All General Motors Light Duty Trucks and Utilities




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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
Changing of tire size on midsize and fullsize trucks and utilities.

Recommendation/Instructions:
When attempting to change tire size, it is important to understand what tire size the customer is interested in installing. General Motors will ONLY support a tire calibration for tires that have been sized, tested and designed for the vehicle in question and its applications. DO NOT use the information offered in the ABS module (tire size selection) as a guide to see what tires are applicable to the vehicle in question. Keep in mind, changing the tire size in the ABS module will only affect ABS wheel speed sensor calibration that are direct inputs to the ABS module. It will not correct or calibrate the speedometer's accuracy, and will not change the calibration of the ABS rear speed sensor value on vehicles that obtain rear wheel speed data from the ECM/PCM/VCM via the VSS.

Example:
2004 Chevrolet Colorado LS with 235/75/R15 tires. The customer would like to install P265/75/R15 tires. Although ABS module shows this tire can be programmed, the 265/75/R15 will only fit under a Colorado with the Z71 suspension. This suspension will offer acceptable wheel clearance. Therefore, a calibration will not be offered for this application.
Example:
A second example would be bulletin 03-03-10-006A, which offers 20" wheels and tires for the full size pick up trucks. A calibration has been developed for this specific application in response to popular demand from our customers. Accessories released an approved tire wheel combination that has been tested and validated by GM Engineering for a specific application.

The only way to confirm that a calibration is available would be to visit YOUR sales department. Using the sales order guide, the following combinations of items must be available for the vehicle in question... If you can build a vehicle, using the sales guide, using the same model year, body style, engine size, transmission type, axle type and suspension type, any tire offered for that combination will be offered a calibration. If your sales guide does not cover the model year vehicle that is in question, please contact Techline Customer Support at 1-800-828-6860. Please do not call Technical Assistance Center for these concerns.

Once the tire application has been confirmed per the sales guide, contact Techline at 800-828-6860 for a VCI Number. Please note, there may be a cost to the customer for this service. This is not a feature that is covered under the manufactures warranty.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.