What am I buying that I do not already have?
Ask the provider to compare your actual product edition and enabled features with the proposed service. Who checks that protection is running, investigates alerts, approves containment, and follows through? A subscription and a named response owner are not the same thing. Government guidance for MSP customers emphasizes clear security responsibilities in the agreement. [2]
Do not assume software you purchased independently is a basic consumer product. It may already support central management and investigation. The provider should identify the specific gap rather than saying its preferred brand is simply better.
Is standardization a legitimate reason?
It can be. A provider may build its technician training, monitoring, reporting, and response procedures around a supported set of products. Supporting an unfamiliar alternative could require different skills or processes. That is an operational explanation to evaluate, not proof that your software is defective.
Ask the provider to distinguish three statements: "Your product cannot meet this requirement," "We have not validated this combination," and "We do not offer support for it." A support policy can be a firm condition of that provider's service without being a universal technical rule.
Why not just run both?
Two overlapping real-time antivirus products can conflict or affect performance. Some combinations are supported when assigned different roles, such as an approved passive mode. Compatibility depends on the products, operating system, configuration, and licensing; two installations do not automatically provide two useful layers. [1]
Have the provider plan the handover and verify protection before removing the previous product. Do not disable protection yourself to settle a purchasing disagreement.
What about the money I already spent?
Ask whether existing licenses can be used, a supported exception is available, or a staged transition is reasonable. Request the remaining subscription cost, migration charges, and ongoing service price in one comparison. Do not assume a refund or credit, or postpone an urgent security correction merely to reach renewal.
A replacement may be justified, but "we only use ours" is not enough information for your buying decision. Request the reason, alternatives, costs, and responsibilities in writing. Apply that same test to a proposal from Blue Tree Technology®: buy a supportable security outcome, not an unexplained second purchase.
Verification sources
[1] Microsoft: Antivirus compatibility with other security products
[2] Joint cybersecurity advisory: Protecting MSPs and their customers