```bash
#!/bin/sh

# Define variables
APP_NAME="ipchanger"
APP_DIR="/usr/lib/lua/luci"
CONTROLLER_DIR="$APP_DIR/controller/$APP_NAME"
MODEL_DIR="$APP_DIR/model/cbi/$APP_NAME"
CONFIG_FILE="/etc/config/$APP_NAME"
CURRENT_THEME=$(uci get luci.themes.Theme_argon 2>/dev/null)

# Create directories
mkdir -p "$CONTROLLER_DIR"
mkdir -p "$MODEL_DIR"

# Create configuration file
if [ ! -f "$CONFIG_FILE" ]; then
    cat <<EOF > "$CONFIG_FILE"
config ipchanger 'settings'
    option new_ip ''
EOF
    echo "Configuration file created: $CONFIG_FILE"
else
    # Check if the configuration file has the correct content
    if ! grep -q "config ipchanger 'settings'" "$CONFIG_FILE"; then
        cat <<EOF > "$CONFIG_FILE"
config ipchanger 'settings'
    option new_ip ''
EOF
        echo "Configuration file fixed: $CONFIG_FILE"
    else
        echo "Configuration file already exists and is correct: $CONFIG_FILE"
    fi
fi

# Set configuration file permissions
chmod 644 "$CONFIG_FILE"
echo "Configuration file permissions set to 644"

# Create controller file
cat <<EOF > "$CONTROLLER_DIR/index.lua"
module("luci.controller.$APP_NAME.index", package.seeall)

function index()
    entry({"admin", "system", "$APP_NAME"}, cbi("$APP_NAME/ipchanger"), _("IP Changer"), 60)
end
EOF
echo "Controller file created: $CONTROLLER_DIR/index.lua"

# Create CBI model file
cat <<EOF > "$MODEL_DIR/ipchanger.lua"
local uci = require "luci.model.uci".cursor()

m = Map("$APP_NAME", translate("IP Changer"), translate("Change the IP address of the LAN interface."))

s = m:section(NamedSection, "settings", "ipchanger", "")
s.addremove = false
s.anonymous = true

-- Retrieve current IP address
local current_ip = uci:get("network", "lan", "ipaddr") or "192.168.1.1"

-- Field 1: Display current IP address
o = s:option(DummyValue, "current_ip", translate("Current IP Address"))
o.default = current_ip
o.readonly = true

-- Field 2: Input for new IP address
o = s:option(Value, "new_ip", translate("New IP Address"))
o.datatype = "ip4addr"

-- Logic for processing the submit button
function o.write(self, section, value)
    if value ~= current_ip then
        -- Use sed to replace all instances of the old IP with the new IP in /etc/config/network
        os.execute("sed -i 's/" .. current_ip .. "/" .. value .. "/g' /etc/config/network")

        -- Calculate the new gateway address (replace the last byte with 1)
        local new_gateway = value:gsub("%d+$", "1")

        -- Update gateway address
        os.execute("sed -i '/option gateway/s/192.168.%d+.%d+/" .. new_gateway .. "/' /etc/config/network")

        -- Replace all instances of the old IP with the new IP in /etc/config/dhcp
        os.execute("sed -i 's/" .. current_ip .. "/" .. value .. "/g' /etc/config/dhcp")

        -- Update ipchanger configuration
        uci:set("$APP_NAME", "settings", "new_ip", value)
        uci:commit("$APP_NAME")

        -- Restart network services
        os.execute("/etc/init.d/network restart")
    end
end

return m
EOF
echo "CBI model file created: $MODEL_DIR/ipchanger.lua"

# Clear LuCI cache
rm -rf /tmp/luci-*
echo "LuCI cache cleared"

# Check the current theme
if [ -n "$CURRENT_THEME" ]; then
    echo "The current theme is argon."
    echo "Note: The argon theme may not fully support CBI components."
    echo "For compatibility, it is recommended to switch to the default bootstrap theme."
    echo "Would you like to switch to the bootstrap theme now? (y/n)"
    read -r response
    if [ "$response" = "y" ]; then
        uci set luci.themes.Theme_argon="bootstrap"
        uci commit luci
        echo "Theme switched to bootstrap. Please refresh the web interface."
    else
        echo "You can manually switch to the bootstrap theme later via the web interface."
    fi
fi

# Output completion message
echo "LuCI application '$APP_NAME' successfully installed."
echo "Access it via the OpenWRT web interface: 'Admin > System > IP Changer'."
```