I'm playing about with a little piece of nonsense that I thought I'd write in the first person, present tense, with the intention of subbing it to the local rag. But here and there I see what I perceive as a need to change present tense to past tense. One such example is changing "For some days I have been attempting to purchase a new phone" to "For some days I had been attempting to purchase a new phone." But then, two sentences further on - after a sentence that simply describes the phone - I want to revert to the present tense, with "Why don’t I move into the 21st Century, do I hear you ask? Simple, really: old farts like me become confused . . ." Is it ok to do things like this?