A basic solution to substitute a straight uppercase I by a crossbar uppercase I

Crossbar_I.fea

# Copyright 2020 JEAN-BAPTISTE MORIZOT. All rights reserved.
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# PUT IN THIS GROUP ALL THE GLYPH NAME OF ALL LETTERS IN YOUR FONT (MAYBE FIGURES TOO, IN CASE YOU USE LEEETSPEAK).
# DON'T PUT THE GLYPH NAME OF PUNCTUATION
@ALL = [A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z];

# I ASSUME YOUR ALTERNATE I IS CALLED I.crossbar. IF NOT THE CASE, USE YOUR OWN NAMING

# ALWAYS SUBSTITUTE BASIC I BY CROSSBAR I EXCEPT WHEN PRECEDED OR FOLLOWED BY A LETTER (== IN A WORD)
# MAYBE THE CODE COULD BE ADDED IN THE LIGA FEATURE TOO.
feature calt {
    ignore sub @ALL I';
    ignore sub I' @ALL;
    sub I' by I.crossbar;
    } calt;
  
# PLUS A SOLUTION TO CANCEL THE SUBSTITUTION
feature ss01{
    sub I.crossbar by I;
    } ss01;

# AND A SOLUTION TO FORCE THE SUBSTITUTION
feature ss02{
    sub I by I.crossbar;
    } ss02;