That command is for django and wagtail based sites would help if you need to convert all your old blocks to new on every page

block_convert.py

import json
import logging
import os
import uuid
from copy import deepcopy

from django.conf import settings
from django.core.management import BaseCommand
from wagtail.core.models import Page

logger = logging.getLogger(__name__)


class Command(BaseCommand):
    """
    That command is for django and wagtail based sites would help 
    if you need to convert all your old blocks to new on every page 
    (ex if you drop ListBlock for StreamBlock - see https://github.com/wagtail/wagtail-localize/issues/355). 
    
    Save it to "<app>/management/commands/", review and rewrite handle_block method for your block, 
    dont forget id field, it is important for some actions with content such as wagtail-localize sync translations.
    """
    help = "Convert source block for every page to target using handle_block method"

    def add_arguments(self, parser):
        parser.add_argument(
            "-s", "--source", action="store", dest="source",
            default="icon_grid",
            help="ex. icon_grid",
        )
        parser.add_argument(
            "-t", "--target", action="store", dest="target",
            default="icons",
            help="ex. icons",
        )

    def handle_block(self, block):
        result = []
        for item in block["value"]["items"]:
            if not item["icon"]:
                continue

            result.append({
                "id": uuid.uuid4(),
                "type": "icon",
                "value": {
                    "caption": item["caption"],
                    "icon": item["icon"],
                },
            })

        block["value"]["items"] = result

        return

    def handle(self, *args, **options):
        logger.info("Convert %s to %s", options["source"], options["target"])
        qs = Page.objects.all().specific()
        for page in qs:
            content = getattr(page, "content", None)
            if not content:
                continue

            content = list(page.content.raw_data)
            content_new = []
            for block in content:
                if block["type"] == options["target"]:
                    continue
                elif block["type"] != options["source"]:
                    content_new.append(block)
                    continue

                block_new = deepcopy(block)
                self.handle_block(block_new)

                block_new["type"] = options["target"]
                # content_new.append(block)
                content_new.append(block_new)

                page.content = json.dumps(content_new)
                page.save()