Adding the reseller ID should NOT impact your existing fleet. Please note, unless you have a good reseller, they will NOT backload your prior purchases. Since you did not have a org ID, they never associated prior sales to your organization. Once you have ABM setup, they can. But you may need to ask them and if they are not familiar with the process, it may take some time. Generally, you may need to treat your equipment as two sets of devices; everything before ABM and everything after. The before can be user-initiated enrolled and the after can go through automated enrollment.
For your old gear, you can ask the reseller to assign the devices to your ABM. Doing so could have impact on devices running Sonoma and above as Sonoma supports retroactive supervision. For example, if you have a machine that was purchased prior to ABM and it is NOT enrolled in device management, then the reseller backloads the assets and you assign it to an MDM and a pre-stage, then the Sonoma device (T2 or better) will eventually query Apple's activation server and prompt the user to enroll via automated device enrollment. They are offered one "not now" and then the device is locked until enrolled.
An alternative to asking your reseller to backload your pre-existing devices is to handle your old equipment via attrition. Each time you turn a unit over (someone leaves and your reissue), use Apple Configurator to side load the unit into ABM. This allow you to control the flow of the old equipment into ABM and thus automated enrollment.
Hope this helps