An example of how to filter the Submit button text used within Event Espresso.

ee-site-specific-plugin.php

<?php
/*
Plugin Name: Site plugin for myexamplesite.com
Description: Site specific code for myexamplesite.com
*/
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */

//This function filters the Submit button text based on the current 'step' the user is at.
function espresso_filter_spco_submit_button_text( $submit_button_text, EE_Checkout $checkout ) {
    if ( ! $checkout instanceof EE_Checkout || ! $checkout->current_step instanceof EE_SPCO_Reg_Step || ! $checkout->next_step instanceof EE_SPCO_Reg_Step ) {
        return $submit_button_text;
    }

    // $checkout->revisit - whether this is the first time thru SPCO or not : false = first visit, true = return visit (ie repay or edit)
    // details for the CURRENT SPCO Reg Step
    // $checkout->current_step->slug();  ex: 'attendee_information', 'payment_options', 'finalize_registration'
    // $checkout->current_step->name();
    // details for the NEXT SPCO Reg Step that follows the CURRENT SPCO Reg Step
    // $checkout->next_step->slug();
    // $checkout->next_step->name();

    if ( $checkout->next_step->slug() == 'finalize_registration' ) {
        $submit_button_text = __( 'Proceed', 'event_espresso' );
    } else if ( $checkout->current_step->slug() == 'attendee_information' && $checkout->revisit ) {
        $submit_button_text = sprintf( __( 'Welcome back. Proceed to %1$s', 'event_espresso' ), $checkout->next_step->name() );
    } else if ( $checkout->current_step->slug() == 'attendee_information' ) {
        $submit_button_text = sprintf( __( 'Proceed to %1$s', 'event_espresso' ), $checkout->next_step->name() );
    } else {
        $submit_button_text = sprintf( __( 'Finished with %1$s? Go to %2$s', 'event_espresso' ), $checkout->current_step->name(), $checkout->next_step->name() );
    }
    return $submit_button_text;
}
add_filter( 'FHEE__EE_SPCO_Reg_Step__set_submit_button_text___submit_button_text', 'espresso_filter_spco_submit_button_text', 10, 2 );