Answer:
Although Summer's lease hath all too short a date, it brings forth beautiful memories and experiences.
To change the given sentence into a complex sentence, we can add a dependent clause to provide additional information or context. In this case, I have added the dependent clause "it brings forth beautiful memories and experiences" to give more detail about the concept of Summer's lease having a short duration. The conjunction "although" is used to introduce the dependent clause and show the relationship between the two clauses. The independent clause "Summer's lease hath all too short a date" remains unchanged and serves as the main clause in the complex sentence.