Increasing challenges due to obsolete systems and components, as well as recent market conditions, have driven nuclear power plant (NPP) owners to increase their focus on upgrading outdated analog Instrumentation and Control (I&C) systems to digital as an effective approach for reducing the O&M costs and increasing reliability. As most operating plants in the U.S. have already extended their license to 60 years (and several are preparing for an extension to 80 years), upgrading outdated 1960s and 70s technology to Digital becomes unavoidable. Digital Systems provide the flexibility of software parameterization, calibrations, and setpoint adjustments. Customization of system functionality through software offers almost endless combinations of engineered alarms, self-checks, and automated surveillance checks. The customizability of application software provides operators with a combination of controls with which they are familiar, as well as the capability of new and improved controls and interfaces. To greatly increase the probability of implementing a successful analog to digital upgrade, adequate planning and incorporation of a project life cycle process for an Analog to Digital Control Upgrade is key to ensuring all required Regulatory and Industry Standards are met, especially for Safety Related Applications. One important planning consideration is realizing and understanding the additional I&C Calculation related Uncertainty Errors involved with Digital Systems over and above those evaluated for analog systems. These additional considerations also provide an opportunity to perform reanalysis of prior drift evaluations using historical calibration data, especially for many of the front end analog sensors, to determine if additional margin can be obtained between the setpoints and corresponding Technical Specification Allowable and Analytical Limits. The re-analyzed drift evaluation may allow for reduction of previously assumed uncertainty error values that may have been based on overly conservative vendor specifications rather than actual operational data. The additional margin realized may allow for extension of calibration surveillance frequency intervals resulting in cost savings that could be used towards the upgrade. Also, when an analog instrument loop is replaced with a digital loop, the sensor is often the only component in the loop that is susceptible to drift; therefore, the channel drift is reduced, providing more margin. Conversely, since some instrument loop components are replaced with digital components (e.g., I/O modules, software) new errors could be introduced into the measurement with the use of digital technology (e.g., errors that could result from analog-to digital or digital to analog conversion or from numerical methods used in the software (e.g., curve fitting)). This paper and presentation will present an overview of the additional uncertainty error considerations involved with Nuclear Plant Analog to Digital Controls Upgrades, as well as describing the method and benefits of reevaluating the existing drift evaluations for the existing components that may be reused as part of the upgrade for consideration of extending Calibration Surveillance Frequency Intervals.