Probably a misuse of Nebula Core functional stuff, but interesting to try

LazyConcatTest.cls

/**
 * @author aidan@nebulaconsulting.co.uk
 * @date 22/08/2023
 * @description For a given Long text field, when it changes append the old value to the new value with a new line (\n).
 * If the field changes to blank/null then the old value remains.
 *
 * Example:
 *
 * Old Value    New Value       Expected Value
 * --------------------------------------------
 * null         Text 1          Text 1
 * --------------------------------------------
 * Text 1       Text 2          Text 2
 *                              Text 1
 * --------------------------------------------
 * Text 2       Text 3          Text 3
 * Text 1                       Text 2
 *                              Text 1
 * --------------------------------------------
 * Text 3       null            Text 3
 * Text 2                       Text 2
 * Text 1                       Text 1
 *
 */

@IsTest
private class LazyConcatTest {

    static List<Contact> oldList = new List<Contact>{
            new Contact(),
            new Contact(Description = 'Text 1'),
            new Contact(Description = 'Text 2\nText 1'),
            new Contact(Description = 'Text 3\nText 2\nText 1')
    };
    static List<Contact> newList = new List<Contact>{
            new Contact(Description = 'Text 1'),
            new Contact(Description = 'Text 2'),
            new Contact(Description = 'Text 3'),
            new Contact()
    };

    @IsTest
    static void twoIterators() {
        nebc.LazyIterator oldDescriptionsIterator = new nebc.LazyTriggerContextPairIterator(oldList, newList)
                .filterT(new nebc.IsFieldChangedInTrigger(Contact.Description))
                .filterT(new nebc.OldFromTriggerContextPair(), new nebc.IsNotNull(Contact.Description))
                .oldRecords()
                .get(Contact.Description);

        new nebc.LazyTriggerContextPairIterator(oldList, newList)
                .filterT(new nebc.IsFieldChangedInTrigger(Contact.Description))
                .filterT(new nebc.OldFromTriggerContextPair(), new nebc.IsNotNull(Contact.Description))
                .newRecords()
                .putIf(new nebc.IsNotNull(Contact.Description), Contact.Description, new nebc.Add(new nebc.FieldFromSObject(Contact.Description), '\n').add(oldDescriptionsIterator))
                .putIf(new nebc.IsNull(Contact.Description), Contact.Description, oldDescriptionsIterator)
                .forEach();

        Assert.isTrue(makeAssertions());
    }

    @IsTest
    static void tuples() {
        new nebc.LazyTriggerContextPairIterator(oldList, newList)
                .filterT(new nebc.IsFieldChangedInTrigger(Contact.Description))
                .filterT(new nebc.OldFromTriggerContextPair(), new nebc.IsNotNull(Contact.Description))
                .mapValues(new nebc.TupleMapFunction(
                        new nebc.IfThen(
                                new nebc.IsNotNull(new nebc.Composition(new nebc.NewFromTriggerContextPair()).compose(new nebc.FieldFromSObject(Contact.Description))),
                                new nebc.Add(
                                        new nebc.Composition(new nebc.NewFromTriggerContextPair()).compose(new nebc.FieldFromSObject(Contact.Description)),
                                        '\n'
                                )
                                        .add(new nebc.Composition(new nebc.OldFromTriggerContextPair()).compose(new nebc.FieldFromSObject(Contact.Description))))
                                .elseFunction(new nebc.Composition(new nebc.OldFromTriggerContextPair()).compose(new nebc.FieldFromSObject(Contact.Description))),
                        new nebc.NewFromTriggerContextPair()))
                .mapValues(new nebc.Swap())
                .forEach(new nebc.SObjectPutField(Contact.Description, new nebc.IdentityFunction()));

        Assert.isTrue(makeAssertions());
    }


    static Boolean makeAssertions() {
        Assert.areEqual('Text 1', newList[0].Description);
        Assert.areEqual('Text 2\nText 1', newList[1].Description);
        Assert.areEqual('Text 3\nText 2\nText 1', newList[2].Description);
        Assert.areEqual('Text 3\nText 2\nText 1', newList[3].Description);
        return true;
    }
}