I have written my entire novel thus far in past tense. However, I feel like my final chapter/epilogue would work better in present tense. I want to give the reader the sense that everything s/he has read so far is a narration of events. What do you think? Is the tense-switching rule so vital that it would take away from the overall strength of my book?
Switching to present tense in the epilogue would suggest that the story is in a frame. That is, the story is a narration in the present of events that took place in the past. The narrator is not relating in real time, but is looking back over the entire story and relating it in hindsight.
This is a common enough technique, but if it is employed only in the epilogue that means that you are springing the fact that there is a frame on the reader at the end, rather than placing it in the frame from the beginning. I don't see why that cannot be made to work, but I think you have to think about it consciously, and make sure the main story is narrated as if it were in a frame, even if the reader is not aware of it.
This may be subtle, but I think if it is done well, the reader should feel satisfaction, rather than surprise, when the frame is made explicit in the epilogue. In other words, the switch to present tense should make structural sense in the book as a whole, rather than coming across as a random change.
This kind of thing is always Your Mileage May Vary, of course, but I think if you're doing it in an epilogue (clearly labeled as such), you can probably get away with it. The main story is done, and this is a separate after-piece.
The tense-switching rule is meant to address comprehension. If part of what you want the reader to comprehend is that "All of this part that you just read is in the past, and we're now in the present," then switching tenses may be exactly what you need to aid comprehension. There's no ironclad rule against it.
You can do whatever you want, there is no law, governing the use of tense (past, present, or even future) in creative writing. Essentially it all comes down to how you are going to handle it: use of the present tense conveys a sense of urgency and often indicates immediate action, which makes writing (and reading) a longer piece somewhat difficult.
Give it a shot and see how it goes--you can always switch to past tense, if you feel that your epilogue sounds strained and unnatural (it is harder to decide what to write, then how to write it).
If you can't tell, give it to a friend to read.
I agree that it's springing it on the reader a bit, but done properly it can be a powerful tool. An example would be a story about WWII told by a veteran. The main text of the story contains the events that happened, in the past, told in past tense - as a story like this often would be. When the epilogue comes and the veteran is talking about missing his brothers, or that there are so few of them left, or something like that, the switch can be extremely powerful emotionally.
If it drives the narrative, an emotional response, or benefits the scope of the story, then definitely go for it - just make sure you do it right! :)
The 'rules' of arts and literature are there to make you stop and think before you break them.
The adverse thing about changing tenses is that it can confuse the reader and break the sense of immersion. Typically if you use the more standard past tense for a narrative the reader stops thinking about it as being paste tense per se and just follows it as a sequence of events and keeping it consistent throughout the bulk of a story maintains the sense of a moving present which is also convenient for cutting out irrelevant passages of time.
Having said that switching to the present tense for an epilogue can also make sense especially if it is a fairly well contained scene.
This is often used when writing something where you want to give the impression that the narrator was involved in the events of the main story and it's often seen in 19/20th century proto-sci-fi/fantasy (Lovecraft, Verne etc ) . The danger here is that it can be a bit of a clunky way to force a twist. But it can work with care, especially if it gives genuinely relevant commentary on the events of the main story.
Ultimately it is up to your skill and discernment as a writer to decide if it works or not but you are doing the right think by asking the question.