[@PlatinumSkink] I'd go with "that", not "this" -- "this", to me, implies it's something present or fairly immediate, and it feels more like a tense/perspective error than anything else, but it doesn't seem particularly egregious.  I would accept "this" here, as a stylistic choice.  I don't remember the specifics of when you use which, but it does make sense.  "This"/"that" is specific, and emphasizes something more than just an "it".  "It" is just a fairly generic sort of word, and if you want to highlight the death, I wouldn't use "it".  In terms of replacement, you should be replacing "that"s, not "this"es, [@Terminal].

I am going to surprise you, but I'm not telling you how this will happen.
Put that in past tense:
I was going to surprise you, but I didn't tell you how this would happen.

...Actually, maybe it should be a matter of correcting "was going" to "would"?  Honestly English is rather flexible, and there's always half a dozen ways to say anything and any one of them might not feel quite right to someone but fine to others.

[@Cruallassar] How many do you have right now?