Learning new phonemes late in life

How hard would it be a foreigner who has spoken English as a second language for say, 30 years, to modify his speech habits? For example, he hears and pronounces the following words identically: "letter" like "later"; and "fit" like "feet". What does he have to do to put an end to his old ways and devise a new way of perceiving these sounds differently?

Is this purely a matter of conscious listening and ritualised repetition over a period of time? IF this is the case, will his new found speech pattern eventually become second nature to him like that of a native? Or will a consistent effort (greater than that of a native) always be required consciously?

Yes, it's a matter of conscious listening and ritualised repetition. It's also a matter of slowing down your speech and being extra careful when you speak.

My pronunciation was bad for the first 8-9 years of learning English. Then I started paying attention to it and studying it consciously, and it gradually became native-like. I occasionally make pronunciation mistakes and I always need some practice before I get to that native-like level. For instance, if I don't speak English for a month, and then start speaking, my pronunciation will not be a perfect copy of American pronunciation (there will subtle differences in the pronunciation of phonemes, I may find some words difficult to pronounce, etc.)
But I believe that if I lived in the US for a couple of months, my pronunciation would be indistinguishable from that of a native.