How can I correct this sentence

Fruit is one of those tricky words that normally applies to a plural, but can also be used with an 's' in certain circumstances. 

Non-countable:

I need to buy some fruit (I have not yet decided what variety, though there may be a mixture - I cannot yet count them)

I like to eat fruit (varieties unspecified so I can't count them)

Countable:

There are many different fruits on the table (different varieties of fruit but you can count them all)

'Fruits of the Forrest' pie (specific varieties of fruit - normally brambles, apples, red currents etc, - which can be named and counted)

For your sentence you need the non-countable version (fruit) because you're writing about all the different varieties of fruit available, without wanting to specify what they are.

For example, if the price of fruit rises substantially, a fruit eater is likely to change over to non-organic versions, because the products are so similar.

I have also made the following changes:

• removed the 'a' before non-organic because again you're talking about more than one.
• changed the third 'fruit' to 'versions' (though you could use 'options' if you prefer) to reduce the number of times 'fruit' is used in the sentence. Too much repetition of a specific word can make your sentence clunky.   

You may also want to change 'change over' to 'switch' - fewer syllables but this is purely stylistic so you don't need to if you don't want to. 

I hope all that helps!