Design plans are just a plan. You can and should change that plan. In fact, this is stated in both Clause 7.3.1 of the ISO Standards, and in Section 21 CFR 820.30b of the FDA QSR. You can make your plan as detailed as you need to, but I recommend starting simple and adding detail. Your first version of a design plan should include the following 16 tasks:

Identification of the regulatory pathway-based upon the device risk classification and applicable harmonized standards.

Development of a risk management plan

Approval of your design plan (1st design review)

Initial hazard identification

Documentation and approval of design inputs (2nd design review)

Risk control option analysis

Reiterative development of the product design

Risk analysis

Documentation and approval of design outputs (3rd design review)

Design verification and validation, and risk control verification

Clinical evaluation and risk/benefit analysis

Development of post-market surveillance plan with post-market risk management plan

Development of a draft Device Master Record(DMR) /TF Index

Commercial release (4th and final design review)

Regulatory approval and closure of the Design History File (DHF

Review lessons learned and initiate actions to improve the design process