When do we need to change the strategic plan for a company or organization?

- When the inputs and/or the assumption based on which the plan is made changes.

- When the feedback for the strategy's implementation dectates a change to the strategy.

In my opinion Change is a reality that every business be ready to deal.
Companies must adapt quickly to changing business conditions, such as new product launches by competitors, changing consumer preferences and evolving technologies.
Changes in economic conditions, government legislation and other factors such as natural calamities, political instability etc also prompt adjustments in strategy.
These dynamics are the external reasons.
The company would consider changing the strategic plans due to internal reasons, many a times it is observed that executives lack ability and commitment to follow strategic plans due to changes in company growth plans, changes in product development plans or changed financial situations.
In such situations strategies have to be re-framed.
As per my experience every product and service has a life cycle therefore it is the Vision based strategy that needs to be strong and thereafter alignment strategic changes should done.
In the nut shell when company starts under performing a strategy plan change is required.

The logical sequence of making a strategic plan is to look into the mission of the organization and then do an environmental scan (internal and external) utilizing known tools such as PEST, Porter's five forces or SWOT analysis to identify enablers and barriers to acheive the organization's mission. The environment in which any organization is working is never static and changes do happen sometimes rapidly in the environment which may impact the organization's strategy and sometimes render it obsolete.

So, to summarize the strategic plan is a dynamic plan and should  be regularly reviewed and updated to reflect the changes in the environement with the aim of acheiving the organization's mission.

As it is known in the management field that an organization has to set3 main things: - Vision - a set of smart goals where this organization wants to be in the market world wide - Mission - how to work an its goals - Values - the ways that to be adopted inside and outside this organization the change of a strategic plan came after the review of the achievement against the organization goals to find new ways to deploy for the organization career development and to advance and amend the previous results.
and this process may need buy new equipment, software and others to cope with the new plan requirements and to have the best performance at the end

I personally believe that this is answered by Law of diminishing marginal utility